I've used MINWAX High Performance Wood Hardener on various wooden rods for years. This stuff soaks in and dries leaving the wood heavier, harder and tougher than it started. Never have used it on arrows, but I'd try it if I wanted stiffer, heavier shafts.
I made a PVC tube instead of the lightweight commercial ones. With a Schrader valve on one end you can pressurize after loading and filling. If you do a vaccuum soak first it works even better. This drives the stuff deeper into the wood. It would give you an almost composite arrow after drying.
FWIW