It's not entirely trivial because for internal messages we don't store them in PGP/MIME format, for performance reasons (e.g. to make it possible to fetch the body separately from the attachments, or one attachment without the others, they are signed and encrypted separately, unlike PGP/MIME which signs+encrypts the whole message in one go). So, if we serve the messages "as-is" it's unlikely that the email client would display them correctly. We're considering trying to standardize (something similar to) our internal format at the IETF, to enable other clients to implement such a format, but again it's not something we can ship overnight.
tbh the reason I expected this to not be a thing is the reason you have given (consequences), hopefully a standard that suits everyones needs comes through at some point. Appreciate the work.