For posterity - our distributor reported that NXP reported the following:
After discussion with the internal team, the internal team believes our LPC5536's ROM code should support FRO trim by full speed USB SOF frame, then the FRO accuracy can match full-speed USB requirement.
There should be no concern on the FRO accuracy, still, we would recommend you perform the test on your devices to validate if this works correctly on your side.