I don't personally have a Rubberslug account, but
here's my opinion.
You said you don't want to take anything away
from current members. That's very generous of
you, but they members didn't have to pay anything
for their accounts to begin with, right?
I think that the current members should be
willingly accepting to the idea of pop-ups,
because if something isn't done, they won't have
a place for their gallery regardless.
And if they absolutely despise pop-ups, then they
should donate some certain amount yearly, half-
yearly, or something else.
Just a thought...