Cheers guys,
I've been learning a bit more, even since posting above. I saw that frames all have to be the same size, but that canvas expansion trick looks like a useful workaround. I'll look into GAP too.

So far, it looks like I might be able to do what I have in mind. (makes a change, I'm usually frustrated by wanting to do the impossible) If so, I'll post the results here.

Thanks all.