@prologic@twtxt.net as a fan of encryption and privacy, I think signing and encrypting messages are not only desirable but needed, I’d like to see it in action. The downside, as some have discussed is the UX pains for those clients not supporting the extension.
On the other hand, Saltpack or PGP are great for messaging/email, I understand twtxt is not for that. In a similar way, Twitter allows private messaging in the same App, but in a different section. Same with most Social Services.

So I actually believe the UX can be quite simple here. And for your 2nd point of clients that don’t support the extension, you could use different feeds for different purposes and separate out your “encrypted social” activities.