Ah, I get what you're saying now. On the downsampling side, correct me if I'm wrong, but my understanding is that resampling in general is a well-solved problem (brick wall included), given reasonable target rates.

In terms of it being easier to filter an orginal signal than to filter a downsampled->upsampled one... my gut goes the other way, but it's something I'd definitely have to look into before I'd comment either way. Interesting question though.