This is a combination not often talked about - but I like to dig deep into characters and I actually think Mihoshi and Ryoko have a good - if interesting - relationship over the course of the series. Their rapport differs from series to series and show to show, so it's worth breaking them down into their componant atoms a little as we go.
In terms of the OVA, Ryoko seems to see Mihoshi largely as a rival figure. She's not exactly someone Ryoko wants to kill - or not so far - but she's also decidedly unhappy with Mihoshi being around. In the first instance, Mihoshi is her natural enemy - the Police Detective - and even though the statute on her crimes has expired, Ryoko is naturally wary of the police. Secondly, in Ryoko's mind, Mihoshi is another rival for Tenchi's affections, so she goes to some extremes to try and get the girl to return to Galaxy Police HQ out of the way. This act may well have been precipitated by her overhearing of Katsuhito's suggestion Mihoshi should marry Tenchi. (This rivalry is also brought into play in a weakish way in the OVA3, when Ryoko sets off to rescue Tenchi from what she thinks is some kind of love match with Mihoshi aboard the Chobimaru).
From Mihoshi's perspective, she really doesn't seem to see the animosity that Ryoko occasionally has for her. Being as sunny-tempered as she is, she just gets on with her life and is remarkably accepting of Ryoko's past crimes...seeming more alarmed by Ryo Ohki's name than Ryoko's. It seems clear that from her perspective at least, they could definitely be friends...she gatecrashes Ayeka and Ryoko's drinking session in the Onsen to talk about Tenchi - a fact not really appreciated by either woman.
In terms of the other series, however, there is a different slant put on Mihoshi and Ryoko's relationship (one which in some ways paints both Mihoshi and Ryoko in a better light.). In the Tenchi Universe series, Mihoshi seems to have got the idea that Ryoko promised to die with her, and under closer scrutiny you can trace this back to the opening episode where Ryoko teases Tenchi about such a vow. Mihoshi is also involved in that battle and it seems quite probable that she understood Ryoko's words as being meant for her. When they are being chased across the checkpoint, it is Ryoko that Mihoshi runs to and despite Ryoko's obvious panic, it is Ryoko that she clings to throughout the rest of the episode. Strangely, it's not Kiyone - though she and Kiyone are seperated, she doesn't seem overly concerned about it. This shows some intelligence on Mihoshi's part - or at least, a kind of logic, since she knows Ryoko is a space pirate and therefore probably the best qualified to sneak through a place such as the Jurai checkpoint. (Unfortunately for her, she's not quite so well adapted).
Mihoshi's fondness for Ryoko allows her to forget the fact her friend is a space pirate, though, on several occasions - including the moment when she is to return to Headquarters with Kiyone. It seems that in Mihoshi's brain, Ryoko can't possibly be the evil space pirate because Ryoko has become her friend - therefore logically it must be someone else that she was hunting down. Logic is an interesting thing where Mihoshi's mind is concerned!
Perhaps the best evidence of their rapport exists in the Tenchi in Tokyo series, where Mihoshi is both at her most silly and her most astute. After the breakup of the Masaki family, with Ryoko returning to her pirate ways, Mihoshi and Kiyone are sent undercover to her base to try and bring her in and arrest her. Kiyone at first is focused only on the chance of promotion, but it is Mihoshi that points out that Ryoko is their friend and something must have happened to make her act this way. Mihoshi goes on, surprisingly, to reason out that Ryoko is sensitive and that it must have been something involving Sakuya. Her words convince Kiyone to try and find out the truth, and Mihoshi's observations are proven right. Even in the midst of this rift, Mihoshi reassures Ryoko, telling her that she hasn't changed and somehow smoothing over the divide that has grown between them. Even when Ryoko believes Miho-Kiyo have betrayed her, she still thinks of them fondly and remembers how they made her laugh. Conversely, when Ryoko believes she has been betrayed by them, she reacts angrily and decisively, as if genuinely hurt by the betrayal of two good friends.
In the films, the rapport between them is often unspecified. Mihoshi doesn't have a big enough role in Tenchi Forever - in fact, I'm not sure she and Ryoko actually exchange two words throughout the whole of the film. In Manatsu No Eve, there is similarly little effort made to utilise their character interaction. In Tenchi Muyo In Love there is a little more indication of it - the same fed up older sister/ditzy little sister rapport that they seem to have throughout the Tenchi Universe series. Good-natured Mihoshi comes in for Ryoko's banter from time to time, but she is quick to forgive and she doesn't hold a grudge.
In the Mihoshi special, Mihoshi's portrayal of Ryoko is (aside from being surprisingly accurate on occasion), fairly favourable when compared with her portrayal of Ayeka. Whilst it's unclear if the Mihoshi special does link in with any of the other canons, Mihoshi is keen to show that she believes in Ryoko and can see beyond her facade as a space pirate. In her story, the pirate betrays that trust, but it's almost more than just a recollection. It's almost like she's really telling Ryoko that - just because you're a pirate, and you have flaws, I still like you.
In a sense, it's Mihoshi's unwavering belief in people that make it possible for her to befriend Ryoko. And although Ryoko is a criminal with a long history, Mihoshi is quite happy not only to forget those events, but to place her trust in Ryoko whenever she needs to. Because of this, Mihoshi is capable of forming strong friendships with unlikely figures - and Ryoko is one of these kinds of friends.