Is there any way that users can create and integrate their own so-called skins for Quest Pro 4's 'player', to replace its standard GUI?

That could mean a straight substitution of the appearance of the existing ‘surround’ (leaving the objects inventory and places/objects windows where they are) or completely rearranging the whole layout, placing those windows in different positions.

See here

This is not something I can offer as part of a custom build at the moment, unfortunately. It might be easier in future, after I've refactored Quest, to embed an ASL interpreter in a completely different application, so it could look however you want it to, but for the moment this will have to wait I'm afraid.