IANAL, but my understanding is that
(a) all the engines presently supported by ScummVM are supported because
the ScummVM team has legal arrangements with the original authors or
because they're clean-room (which might be the case wrt Sarien).
(b) dependencies matter: If FreeSCI was to depend exclusively on _clean_
ScummVM modules, then it _should_ be possible to avoid any legal
and practical problems by forking off again and copying those clean
modules. (Of course, I'm only using logic, as opposed to the letter of
the law, to arrive at this conclusion.)
I strongly recommend you to look into the legal questions first-- if
_either_ of the above assertions is satisfied, then there should not be a
problem. But note that you guys don't want to be sued by over-zealous lawyers
either...