[whpalmer4]

Problem is that forwarding mail from the iPhone gets you non-plain-text messages, which won't be successfully handled by OF...

Otherwise, I don't think there is anything you would need to do besides configuring OF to handle the +omnifocus case -- any mail server that plays ball just ignores the "+<whatever>" bit when delivering, no need to set up an account, as it is just going to whatever account you've put to the left of the "+" in the address. I use it with my Mac.com and gmail.com accounts without any trouble or extra configuration.

Bottom line: you can send mail directly from iPhone to OmniFocus. Forwarding mail will likely be problematic, and should instead be handled on your Mac. I'd be happy for anyone to prove me wrong on the latter, however :-)