I like the idea, but it's not something I know SMF well enough to implement myself. I could try experimenting with some of the mods out there - would have to see if they work well with the various customizations. The first one I found also seems to have a limitation that it can only save drafts for new topics, not replies (but perhaps there are others that don't have this limitation - I didn't look too hard yet). The other concern is that most of these are quite old now.
Alternatively, SMF 2.1 (which is in an RC phase at the moment, though has been for a long time now so is probably getting very close to stable) has native support for drafts. I don't know what would be involved in upgrading though, and I'd have concerns as to how well it would play with some of our modifications - in particular, PM attachments and spoiler tags.
