Great information. Of course, trying to point out to a web admin that they're violating the RFC is just never going to work either ...
Reading through both of the RFCs mentioned in the wiki article, I may be missing something, but it doesn't look like they specifically address how to handle what DH calls 'wildcard addressing' ... the adding of a '+' and additional text that still goes into the same email box (DH wiki article linked earlier).
If that's the case, DH supporting additional methods would not be against any RFC and would just make it easier for their customers.
I don't expect them to change it just for me, but I'm sure there are others with similar problems. By allowing additional separators in this approach would allow me and others to effectively get rid of our catch-all addresses and should make an easier load on the DH mail servers... With the current solution, I've just run into too many websites (some of them very big names) that won't let me use the '+' sign.