The Speed Shoes music is hardcoded to interrupt the level's usual music. There is a level header parameter that disables this, but only with certain audio formats (none of which are the ones SRB2 itself uses). It might be possible to interrupt it with Lua, but from what I recall, this part isn't fully exposed as of yet.
The game already does this in first person mode. I'm a little anxious about taking up so much screen real-estate with opaque icons unless absolutely necessary.
Funnily enough, every SRB2 track has a BPM defined in the metadata, because the sound test uses it to alter the speed of the boombox animation. Maybe you could just read that data and play additional backing drums at the correct speed?