Yes - getting an idea about *why* something is taken AU would be useful. For some shipfic it's probably self-explanatory (i.e. everything involving Sirius that has actually a future element), but as you say, it'd be instructive (I've been sneaking away from fics ANs announcing they leave out HBP because it exploded their pairing/character of choice).

For shipfic, though, it probably wouldn't make any difference, as extrapolating from old canon would probably give more possibilities, and widening the possibilities is always good in shipfic, especially for rarepairs that weren't being thought of a few years ago.

Any interest in looking over a longish R/L featuring my version of your brilliant 'encounter at Quality Quidditch' and owing quite a lot to Amanuensis's 'Droit du Seigneur'? Just asking, no pressure - as I said, it's long and wobbly and not your cup of tea...