My assumption is the game already looks for a character string at the end of the texture names (e.g. The last U of UNIT9999U.dds if the region uses U regional textures).
My idea is to define a custom string (or multiple strings in a prioritised order) instead of locking it to worldavail to have more options.
The "Z5" is just a completely random value I used as an example. It could just as well be "Cookie" or "Balthagor".
I just like the idea keeping things more orderly and you could simply just give each region using region group Z a random Z[number] primary texture.
The reason I suggest multiple prioritised strings is that it would offer the option of a more generic fallback without requiring duplicate textures (e.g. no need to have 10+ copies of the current Z regional texture to cover the 10+ regions in region group Z).
In the above example Z5 could be say Iraq so if it had a design using model 1000 the game would check if model 1000 have a regional texture for Iraq. In this case the texture UNIT1000Z5.dds.
The fallback would be that if there where a UNIT1000Z.dds but not a UNIT1000Z5.dds it would still use a better texture (in this case likely with desert camouflage instead of maybe flat green)
You could also define say a primary regional texture and use the worldavail choice as a fallback if there is no primary but that would not solve problems with regions like Ukraine unless you ensure it have one for all relevant cases.