Hmm, well, I've been in touch with a company in the Netherlands, http://www.nedsecure.nl/ who offer pretty much what you're suggesting.
Transparant email encryption through a webserver/webmailportal. So you can send encrypted messages to your clients, they get a notification mail, and can check the mail you sent online. Bit cumbersome perhaps, but well, technology probably improved from back in nov-2005 that I saw these guys at some InfoSecurity convention.
Unfortunately, the site seems to be dutch only, but dutch ain't too different from english, with some imagination