Using Walmsley as an example. He played amateur and uni RL but was outstanding at every senior level. He was NCL player of the year, which got him the move to Batley, and then Championship young player of the year, which got him the move to St Helens. However, how many slots do these clubs have for "project" players, which is what combines, the like of which the thread starter has proposed, will throw up.
I can only speak for what I see regularly, which would be the London Men's League. In my view, there are semi-pro level players in that competition. There are players who went through the Broncos academy, played competitively in other areas or high-quality converts but they're not Championship 1-ready. Do the semi-pro teams have the time and money to turn, say, a 24-year-old into a C1 player for them? Probably not.