I've been experimenting with different forms of protected headers.

I tried modifying the Multilayer PGP/MIME Message with Protected Headers and Legacy Display Part , which has this structure:

A└┬╴multipart/encrypted
B ├─╴application/pgp-encrypted
C └─╴application/octet-stream
   ↧ (decrypts to)
D  └┬╴multipart/signed
E   ├┬╴multipart/mixed ← Cryptographic Payload
F   │├─╴text/plain ← Legacy Display Part
G   │└─╴text/plain
H   └─╴application/pgp-signature

I'm attaching here two different versions of this structure.

The "normal" variant comes directly from draft -02, following the recommendations there:

The "inline" variant where the original body (part G above) has an additional header of Content-Disposition: inline:

(i'm experimenting with this "inline" variant to try see whether it is an improvement for other systems, see the screenshots project).

However, the "inline" variant actually doesn't render part G at all. Here are the two screenshots:

normal:

I'm not sure what it is about the addition of this inner header which makes Outlook appear to swallow the internal part.