News to me, cuz I didn't test it properly, but you CAN include your own URP shaders in the asset bundle and the app will load them with no problem.

In the images above, I created this test shader that is only an emissive red color and applied to a copy of my avatar_model. I include the shader in the asset bundle and then I build it. It loads in the app!

That opens infinite possibilities for you to color your model the way you always wanted, for example creating outline shaders, toon shaders, procedural texture shaders, WHATEVER YOU WANT. Pretty cool tbh.