Hmm, if there actually is some way to jump into a midi, you could measure the BPM of the "original" music, and set a timer when it original midi starts to play. Whenever the timer hits certain key points, (e.g. the time corresponding to eight beats) you can check for conditions to change the music. I haven't used RPGMaker for almost two years though, so I don't know what is possible to do these days. If RPGMaker has a modulo operator, it would help a lot.

If you can't jump into a midi file, start off with something really repetitive and switch to a more complex piece?

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram