My post doc hours were all accrued in such an "informal" manner. I had a staff position (e.g. real job) where I was given supervision by a licensed psychologist. In each setting (private special education school for children with ASD and school consult services out of a hospital based specialty pediatrics clinic) my services were mainly billed to entities that did not require a license, but with responsibilities in line with those of a psychologist. I did a smattering of clinic based assessment and social skills/Barkley Defiant Children groups (not sure how or these wee billed). I'd suggest making sure that such an arrangement meets requirements of any jurisdiction you want to be licensed in (which I did), as well as have some sort of training plan/ contract for supervision in place (which I didn't do, but didn't have any issues with, fortunately!).