It changes the mechanics at least. A quick glance at RFC(2)822 makes it clear that this is a pretty weakly specified behavior and is somewhat of an edge case. However, rewriting the envelopes has become somewhat prevalent since strict DMARC has been adopted and I suspect that most recent MUAs will handle it well. I know that at least with gmail it works as I would expect. (Makes sense considering that this is how Google Groups handles the problem.)
In any event, I really think it's worth a shot since having the subject and footer tags is valuable. If it turns out to be problematic, it's not the end of the world and things could be easily switched to go the lkml route...
--adam