Well, in CB-Emu the Fidelity Mach-II C+ v2 is executed directly (i.e. by CB-Emu itself) and of course doesn't use any chess plugin.

I've tried it now myself and everything works fine, so the bug must be on your side.
If you make the promotion move (with this Mach II), then immediately after the pawn move the display shows 'Pr' and waits, until you press one of the piece buttons (Q/R/B/N) - after that the pawn is (automatically!) replaced by the selected piece, and only then the Mach II responds with its next move (even if it's a ponder hit).
So what's actually your problem with it?