Ars Technica allows 30 minutes, I believe, and it doesn't seem to be abused. People that reply will quote the bit they reply to so it's clear what they refer to anyway.
So how about 30 minutes editing window, and a quick, one-button-press to quote the parent post? Just to encourage people to include the original bits in their replies?
For added protection you could colour the edited text in dark purple, say, just to make it clear to people what has been edited?