morefunc.xll can be embedded into the file itself I believe so it does need to be installed directly on all clients using the file (I believe there are issues using embedded feature in XL2007 or at least I seem to recall having read that somewhere)

re: Sumproduct, you can use VBA obviously to generate a listing of worksheets in the file at any given time and in turn use Dynamic Named Range to increase/reduce in size as required but I would say if you can use morefunc.xll that's not a bad way to go (IMO).