I am completely innocent of rfc1812, and have been out of the game for a long time, but I am pretty sure I was taught (and in turn taught) that a router would reply using the address of the interface that originated the reply unless that interface was unnumbered, in which case it would reply from the loop-back address.
Never too old to learn something. -- "Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid." --Albert Einstein From Larry's Cox account.
