PINE is very easy to use. It will be available soon for personal computers to use. That part of the solution is almost there.
That part of the solution is already done. There are already several very good POP/IMAP clients for Macs and PCs (Eudora, NuPOP, etc). Why the fixation on a particular mail agent? There is no way that you are going to get people to agree on a single MUA, therefore it seems that the comm channel is the beastie that one should focus on for encryption.
How do we get BBS's to use IMAP? they could support IMAP in a similar way that they support Zmodem. What needs to be done is to write some code that does IMAPD functions that could easily be incorporated into a BBS program, and figure out a way for end users to run PINE from their favorite bbs program.
I hate to break it to you, but there already exists a protocol for off-line reading of mail and news over serial connections: QWK. While a noble effort, I sincerely doubt that the BBSers and CI$ users are going to jump over to a completely new protocol for transport of information for off-line reading unless it offers them something that they do not already have, and IMAP/POP just doesn't do that. If one were to be able to offer encrypted TCP/IP connectivity though, then you would be offering people the additional functionality of this comm channel (telnet, ftp, gopher/www, etc) to entice them to switch over.
(and get PINE people to allow for a serial-line connection *or* write a false-packet driver that just strips off TCP/IP headers sends the data over the line and sends back ACK's to the TCP/IP process).
Why not just get them to support IP? Probably easier... All they need is a slip/ppp driver on the host, then you can do the encryption over comm channel and avoid wasting time encrypting something that doesn't need to be encrypted. Many BBS systems are beginning to wade through the shallow water of the Internet, if we had the ability to offer them modifications to provide encryption to thier IP connectivity while they are still new to the game it would be much easier to get them accostomed to the idea that such traffic should offer encryption; not that I think this will happen, but in an ideal world... jim