Its definitely come up! I think it would be cool to have a community working on the translations to MJML from Lexical. There are so many corner cases and email clients out there, it's definitely a challenge to get things working everywhere. What type of compliancy problems have you run into?
I would really like to eventually open source core parts of Loops.
I think parts of the editor makes sense, including the MJML. We spent an entire weekend extending MJML for Superhuman and it would be cool to release that work some way.