To be very honest, I think not much people will need this feature. However, all of our institute members will certainly install the app if this feature gets introduced.
Our Institute has its own email server that is accessible through a roundcube web interface only. So we cannot integrate it in native mail client apps easily outside the institutes machine. However, our Institute does provide a gateway via port 2222. So what I do is like:
I would very very much appreciate if ThunderBird Android app can also provide Proxy support / ssh localhost tunneling support during mannual configuration of emails.
Footnote: I believe building the HTML compose support is already underway.