For copyright/licensing type of reasons I can’t just apply some patches thrown at our side. With some extra paperwork that can be doable though.
Now, would be nice indeed to allow more custom email addresses and aliases. However I think the general case should be something like accounts settings having UI for adding an deleting them, and then the email composer just choosing between what’s been set. Typing the custom address for each composed email sounds quite error prone.
Quickly testing alternatives, on desktop Evolution does something like that. Google allows setting more emails address on the web UI too, not sure right now how the email client would get that list.