Is there any use for the bark trimmings of dry aged beef?
I don't have experience with the umai bag, so I don't know what it produces, but I have dried aged beef in my curing fridge.  There is nothing unsafe about the trimmings (unless, as a comment points out, you contract a bad mold).  Using them partly depends on how long you dry-age, and how desiccated the exterior is.  It could be too dried to be pleasant, and you also don't want any rancid or off flavors, but many folks add it to the grind when making ground beef.  Of course, primal cuts of best quality beef (which, if you are going through the trouble, is what you should use), should contain a large fat cap.  That fat is very useful.