I doubt I'll attempt to add that functionality, the simple work around here is put everyone's login in LDAP as an additional email address.
I'm sure it wouldn't be well received, it's sort of a dicey idea. Certainly open to abuse (put someone else's address in there, or a typo, etc.), and sieve doesn't have the concept of "safety-on vs. safety-off", the safety is always on in sieve, otherwise it could be an "administrative option" and not an "end-user option".
I'm not sure how the vacation functionality is implemented; but if it goes through your normal MTA instead of attempting a direct SMTP connection; then you should be able to have your MTA rewrite the From address on the way out. This is fairly trivial with exim; I'm not sure how hard it would be with other MTAs.
-Pat
