I don't know about your virtual airline since I don't know what sort of simulator information tracking they have, but whilst it is true that you can't use slew mode on VATSIM, I don't see how they would know you were doing it since you aren't moving the airplane itself, but rather an airplane icon when you do a quick pushback. Your plane will still be sitting at the gate. The only other way I can think of is if you're absolutely against it going in slew mode, if you already know what gate you want to push from and which way you're going to push, before you start your program for your virtual airline, position the pushback position in quick pushback (you can abort immediately), start up your software, and then do the pushback. It will have saved the position of the quick pushback, so when you do push from the gate, just select saved quick pushback position (or some variant of that) in the pushback menu.
