In its current form, it actually supports inline but not PGP/MIME. It proved simpler for us to protect an inline PGP encrypted block than to implement a PGP/MIME parser.
Is PGP/MIME planned? I find it less intrusive and IIUC it can encrypt more of the message.
Yep, it's one of the places we'd love to extend this plugin too.
We'd also love help building this open source component since we're not 100% crypto experts. Its' really important to get this right!