First of all it´s interesting to see people provocating regiments when they lack arguments, intellect or anything worthy to say.
Anyway it´s true what Carolus says, there should definitely not be a weak rule about this which you can bypass easily. Nevertheless I don´t really get the point of just playing as an invite. If you don´t care if someone invites for a regiment (e.g. cav) and still plays for a competetive member of the RGL, why would it be that questionable if he´s just a regular member of the cav regiment? The outcome is pretty much the same, just a steam group or a roster or anything else. Vice versa the same, just being invite but still playing 1 event per week or something, it doesn´t matter in the end. I see your points but as long as the membership in said Cav regiment doesn´t affect (and with that I mean double regging in the sense of being in two RGL-attending regiments) the RGL itself, I do not see any kind of problem.
As several people already stated, the practical enforcement would also be rather hard.
But finally calle is right in the point that if a decision is taken, it should be no half shit.