But, if you are a copyright holder of the GPL'd add-on, then you can distribute the combo, simply because even if such action were a violation of GPL, you are not going to sue yourself.
I would like to dispute that affirmation uniquely on an intellectual exercise basis. (assuming, of course, that I still have an intellect to exercise)
I agree that only the copyright holder, you, would be able to sue you(relf) for copyright infringement if such a combo is not legally possible (one possible situation stated in the parent post).
But it's being sued by the Copyright Act the only concerning?
There's a strange law around here (my country loves being ruled by such laws) that states the you are not entitled to waive some guarantees. If you send me legally a copy (no matter the reason) of a software you made stating what the thing does, and by using this software the thing doesn't do what is expected and somehow I was in prejudice, I'm entitled to sue you.
Ok, my country is nuts (no argument here). But I doubt nuttiness is something inherent to my fellow citizens.
There's no chance that you could be sued by some other means?
All your end users are not allowed, under the GPL, to further distribute the combo as expected, and since the GPL can be not applied when distributed under GPL terms violation, chances are that your end users doesn't have even the usage rights about your software.
By knowingly and willingly proceeding in distributing such combo to your end users, there's no situation in which one of them could, rightfully and with the support of the Law, sue you?