Thanks Cammy and Megaboz for all the useful info!
In theory PSX64 will work fine with Tracker Hero when using the PS2 guitar if in the game you turn on the Dance Mat Mode (Not the Guitar Mode). This is because PSX64 maps guitar buttons to directions and in the Dance Mat Mode directions are mapped to the game actions.
The real problem could be that the directions are not matched in the same order, making it difficult to play. But this issue is easy to fix in the code adding another control option.
Megaboz or anybody, if you know the exact mapping of the guitar buttons to the pad directions please write it down

Thanks!