I think that the preclear function already has very limited usefulness, other than a few niche applications like prepping a drive and leaving it on the shelf for rapid expansion.
What you are speaking of is expanding the testing portion to be more thorough and useful. I submit that preclear is the WRONG place for this.
I would much rather see the preclear operation pared back to ONLY a single pass of writing and confirming zeroes, then applying the secret sauce.
Please consider writing a plugin specifically for testing disks, with various levels of thoroughness. I would much rather get behind an unraid disk test suite than continue to hear about people saying preclear is needed when replacing an existing disk. Better to separate the functions to what they really should be, preclear for the very VERY few times it's warranted, and optional thorough disk testing for every disk that is introduced to the array.
You could get quite fancy and write a mostly non-destructive test that includes writes, by reading a sector, flipping all the bits, writing the sector, reading to confirm, then flipping them back, then reading them again. For extra points flush the disk cache after every operation.
It would take days on a large drive, but you could be very sure the drive was good.