Not entirely related, but Delta Chat has been tested with ProtonMail IMAP/SMTP bridge. The result is unfortunate, ProtonMail bridge corrupts MIME structure of outgoing OpenPGP messages and strips Autocrypt header away:
So even with ProtonMail bridge and Delta Chat client running on your desktop it is currently impossible to have encrypted chat with ProtonMail address and encryption. MIME structure corruption is recoverable, but we need to either fix the bridge to keep Autocrypt header, if it is possible with ProtonMail API, or find a way to exchange keys via attachments.