Using MX as the pointer to the other RR

At the INBOXEVENT, I had an interesting talk with a gentlemen working 
for an email hosting provider.  His company hosts email but rarely DNS 
for their customers (the split was something like 2,000 domains that 
they host vs. 65,000 that are customer hosted).  As I recall, there are 
a couple of spam-filtering companies that do this too (MX redirection). 
 His comment was that it is very difficult for them to get the 
customers to correctly add/change MX records and fears that a MARID 
record would be much more difficult.
So he had an interesting idea.  He suggested that instead of looking 
for the MARID record in the domain of the sender, look for it at the MX 
target of the sender.  This solves his problem since his DNS zone is 
the MX target.  This also leads to a solution to the wildcard issue.
Comments?

-andy