You pretty much need 2 regulators to run a carb off an EFI pump. The EFI pump can't dead head, it has to flow. So you will need a bypass type regulator. I have doubts about a bypass regulator regulating such low pressures accurately, so a carb style deadhead style regulator to get the final pressure right.
I would be tempted to do a trail fix by putting looping the supply and return together and put a section of 1/4" line to make some backpressure and use a carb style regulator.
Years ago I got a Holley Projection. Fought some install issues (bad ground it turned out) and wanted to keep a carb with me. I was using a low pressure lift pump and accumulator. So it would have been easy to just move the low pressure line over, defeat the high pressure pump, and run off a carb. During my EFI fight I was doing that and had the ECM running the fuel pump. Once I got the EFI dialed in, never used it. The carb dried up and got filled with dust riding in the spares box. Completely un-necessary.
There is an even simpler way of feeding a carb, gravity. May not flow enough for max HP, but a fuel tank draining into the carb will provide enough to get you out of places. Not pretty, but very simple. Most carburated motorcycles, Model T, so simple.