The goal is to have the S/MIME user experience also for OpenPGP. By that I mean that for S/MIME my users are used to sending a signed mail and after that encrypting mails works. I have received mails like "Here is my key" which were just signed, no attachments. Because with S/MIME the pubkey and the intermediate are part of the signature. It works and is useful.
For OpenPGP the idea was to use "auto-key-retrieve" but that depends on keyservers with all the problems related to that. So I was thinking about adding Autocrypt style headers in GpgOL or attaching the public key as part of a multipart/mixed. But I don't really want to attach the public key even to unsigned mails as I think that this would result in a not optimal user experience. I want all mails to be signed because only then can I really track communication history etc.
Werner said that it's possible in OpenPGP to also put the pubkey into the signature. So this will be our solution to this. The nice advantage is that this will also work for files. :-)