For years SCSI in most of its modern flavors allows passing relative command priority (0 - default, 1 - highest, 15-lowest) from initiator to target. SATA also allows two levels of priority (normal and high). I am not aware yet of any SCSI disks supporting the priorities at this moment (haven't looked very hard), but some SATA HDDs and SSDs I have report it as supported, and at least for some WD REDs I am able to measure the effects of it.
In this patch I've added priority field into CAM SCSI initiator and target command structures (should not break ABI due to alignment), added its support into isp(4) as target and initiator and mps(4), mpr(4) and iscsi(4) and initiator. For SATA drives the high priority is just a single bit in NCQ READ and WRITE commands, so the change is minimal. Into GEOM BIO I've added just a single LOWPRIO flag to not break the KBI, but I can extend it to separate field if people think we may benefit from passing more priorities.