By the way: how secure is "forward to {Nirvana,OneNote,...}" for processing?
Actually, I can answer my own question: probably not secure at all.
Standard email systems have little in the way of authentication on email. So, unless you restrict yourselves to ascii text - not ascii text that looks like MIME, and certainly not HTML - it's a risk.
If your email client can apply a Private/Public cryptographic signature to email you forward to {Nirvana,OneNote,...}, then *maybe* I would trust it.
BTW - for real, this time - I used to do such forwarding all the time. I really miss it. I would hope that there are email clients and mail gateways to GTD apps that are properly secured in this way - but I am afraid that I don't know of them.