Im helping a friend with a pair of 900hp blower motors. Is it acceptable to run the return hose from the back of the fuel rail to the filter head instead of directly to the fuel tank?

That's how it's done on a lot of boats from the factory but can lead to vaporization/vapor lock/hot fuel issues so many people change it to return directly to tank to prevent hot start issues.

I'm doing it with 600 and I know others that have done it up to 1100HP with no issues. Others say not to do it, but really don't give the reasoning behind it. Maybe vapor lock but I believe that can be addressed in the plumbing.

If you run a fuel cooler it's a moot point. Some say the cooler should be after the pump, some say before the pump, but I think it makes more sense to let your fuel line route and raw water plumbing dictate the location. Having a cooler in the worst location is better than none.
With "big" pumps like aeromotive 1000 they are typically overkill for anything short of wide open throttle. A PWM pump speed controller is well worth the couple hundred bucks or so. It runs the pump at varying speeds depending on engine rpm. You can set an idle/low speed setting, a cruise setting, and a "go get em" setting.
The end result us thar the current draw on your batteries/charging system is greatly reduced, fuel pump noise and fuel pump heat is reduced, the fuel pump lifespan is increased, and less pimping related heat goes into the fuel in the first place.

Running it back to the tank helps to get that hot fuel back to the cool tank. If you have a chance to do it I would, everything else seems like a band-aid for hot fuel and dealing with it .

Eddie Young ran myjne backto the tank. If if is good enough for Eddie.......

Padraig

I thought it was not CG approved to run gasoline back to the tank, only diesel. I could be wrong, but all of the CG regulations I have read only states diesel.

I did not read it, but here are the CG rules:
https://www.uscgboating.org/assets/1...YC.1002.01.pdf

Btw: i believe , a good amt of ski / wake boats have return systems.??? I May be wrong.

I have always ran them back to the tank without issue. I had a 454 mag MPI years ago that would vapor lock almost every time it was hot outside. I pulled the return off of the filter head and ran it back to the tank and it never happened again. That was enough to convince me.