To emulate a flushing SSD, the signal really needs to go directly to the SSD firmware so it can decide which is the last OS write it can accept while still having enough power to persist all write and flush requests it has already accepted.

Getting all that right sounds so hard it is probably better to just have enterprise SSD's have a built in supercap to give 5 seconds or so of power to do all the necessary flushing, and for laptop/desktop grade SSD's they only need to offer barriers for data consistency. Laptop and desktop users don't care if they lose the last 1 second of data before a crash as long as what is on the drive is self consistent.