I think that the best way to go would be to record a rhytm pattern that matches the midifile, both in style and in tempo.
Remember that George Michael song that incorporated a sampled groove played by James Brown's drummer Clyve Stubblefield? Maybe not, but anyway my point is that James Brown has been one of the most exploited artists, because his old records are full of exposed grooves (i.e. not covered by other sounds) played by his drummer. That's the reason why today nobody is playing a drums or percussion solo on a CD anymore and, if they ever do that, they always make sure that there is another sounds layered -like a brass or a synth pad- to avoid being sampled and used by some rapper.
(Imagine being able to sample a percussion pattern from a Santana CD and use it synced to a latin style or midifile!)
Aside from that, you can find commercial CDs full of rhytm grooves suited to be looped; all you have to do is sample one that you like and that matches the tempo of your midifile and then sync them together; if you isolate the sampled groove of one of the Ketron styles and listen to it carefully you will understand what I mean: there are styles where they sampled and synced just a drummer playing a riff on his hi-hat, in a crescendo-decrescendo pattern, very difficult to emulate in a sequence.
Other styles have just a sampled noise, that however greatly adds to the overall realism of the style itself (for example, I think that in "Flamenca" they sampled just someone clapping his hands the way a Mariachi player would do).
Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.