My favorite shirt, a beartooth hoodie, is really getting worn out. To the extent that sewing has become a nightly ritual. Has anyone built a hybrid hiking shirt by "cutting-in" a synthetic to the high wear areas of Merino?

I would take a light nylon supplex and just layer it over the high abrasion regions (I'm assuming shoulders and elbows for you?) You could probably get a stretch nylon but depending on the area may not be needed.