The main reason to stuff the frames is to give the ivy something to connect with - roots and sticky patches. If you were to leave the ivy to scramble you'd have to tie in each piece.

I'm wondering if something such as Lonicera nitida would be a better choice than ivy if you want to plant it directly. It will grow through a frame and spread out, may grow more quickly for you, back-buds pretty well - and has the plus of being small-leaved which may give you better definition for your proposed topiary as you prune and shape.

Thanks! The Lonicera sounds like a good idea.