Most/all of the Denso pumps have an integral check valve to prevent bleed down, and according to the Supra FSM they're supposed to hold static pressure for at least 5 minutes without bleeding off.
If your fuel rail pressure is bleeding off quickly, it could be this check valve that's failing, or an injector leaking, or a bad o-ring somewhere, etc., or like you say, the FPR leaking, although if it's a new FPR, that's not as likely as the other two possibilities.
Although it might be a PITA, you could remove the fuel rails and injectors and pressure test the assemblies individually, or together, with air at 15-25 psi and try to find the leaks with soap suds.
If you can eliminate the rails & injectors, then you can be pretty sure it's the pump.
