Your only other option right now is to make a custom troll in the editor and use it in your campaign as an add-on.
This is definitely one of those quality of life fixes that would be great to have and I can't imagine that exposing the XP earned variable to the GM stat editor would be that difficult.
One of the devs would simply need to add a UI input field into the GM monster editor pane and pop the "xp earned" var in there. I don't think Larian is crazy enough to hardcode XP earned values so unless the var is set to private for some crazy reason... This should be a relatively simple feature addition.
That said, I don't have any special insight into the DoS2 class architecture- I'm just making assumptions based on common/standard NPC implementations.
