If by aftermarket you mean Performance (i.e. non-stock), then you are correct, you cannot (and never have been able to) use aftermarket performance motor mounts.
If you mean aftermarket (as in Napa, AutoZone, etc), then as long as they are the exact equivalents, you are fully allowed.
The engine stay rule, as I interpret it, is essentially an additional mount that you can put where you want, except for replacing a stock mount. Check out the IT forums at http://www.improvedtouring.com/chat2
for more information (and faster responses).