I don’t think the issue is on the rails side, since the UI wrongly lists the allowed tags (so that list is wrongly hardcoded somewhere) but %{base_url} is parsed just fine otherwise (see the thread), i.e. with rails i should be able to force save the value (bypassing any checks).
But all this is offtopic, really: my original question was just for a pointer on what rails object to target. If you can only offer paid help then I’ll wait hoping someone else is willing to offer some free help (I did that in other threads, also some github PRs). No hard feelings 
Edit: You edited your post while I was typing. I’ll see how far I get with what you suggested (not sure how to adapt that for my specific example yet). TranslationOverride might be enough of a pointer