Sadly, there are a non-trivial number of MTA installations whose implementers or operators, having discovered that they had not seen a legitimate use of the percent hack in years, decided that they were about as likely to appear in legitimate messages as source routing and dealt with them accordingly. Put more simply, a "%" in a local-part may be least as likely to get a message rejected or dumped as a badly specified DMARC record, so the one is really not a very good cure for the other.
Since the percent hack became a famous vector for open relay abuse, so we all stopped honoring it. A lot of MTAs still reject anything with a % saying something like no more source routing. Mine does.
So this would require inventing something with the same semantics as the percent hack, but a different syntax. Perhaps we can use an exclamation point.
Regards, John Levine, ima!johnl@xxxxxxxx Please consider the environment before reading this e-mail.