I think the regs do address it. 1.413-2(c) says that the exclusive benefit rule will apply to a MEP "in the same manner as under section 413(b)(3) and §1.413-1(d)" (e.g., collectively bargained plans).
1.413-1(d) says (emphasis added):
I read this to say that forfeitures arising from contributions by one employer can be freely allocated towards participants of another participating employer. So there is no need, under the regs, to track each participating employer's forfeitures separately.
That said, there is nothing that says you can't track forfeitures separately, and the participating employers may prefer to do it that way for their own purposes.
Whichever way they want to do it, it might be wise to be explicit about it in the plan document.