after a night sleeping over that, it is simply impossible. i was sleepy yesterday evening, thats why i did not think about the process and how the engines in the backend work (e.g. thunderbird when it tries to access the xml). when i access http://mydomain.com/.well-known/autodiscovery.xml, there is no js executed at all, so this is simply not possible. even if i could point thunderbird to autodiscover.html which would theoretically execute js in a browser and print me the raw xml, i dont know (and i guess and hope not) if thunderbird executes that so it gets the xml.
in that case i’ll recommend her to switch to another hosting solution that allows such stuff. as those requests regarding this were made 4 or 5 years ago, and there seems to be no tracking available nor any progress or need to implement those configs, therefore i highly recommend her to take another hoster.