I think I remember seeing this attempt somewhere…
If I remember correctly, the solution was to use a custom tax query on the category.php template, but querying the taxonomy (crosspostcategory) FIRST in the “or” statement…
So you’d check for the taxonomy first, and then query (tax query) the category taxonomy next in the OR statement.
I’ll look around to see if I can find it.
Since the category.php queries the actual category taxonomy first, I think you have to sort of “trick” the function you use with pre_get_posts. Lol