I think you are right to be concerned. From what I have read if the iCloud Find my Mac wipe/lock was activated on the machine that drive was in, there is a file placed on the drive that locks the drive if it is put back in that same machine. You can see it in
this post. What I recall reading is even if you manage to bypass the EFI (firmware) based iCloud PIN lock, you need to remove that lock file from the drive otherwise the lock will be reapplied if you boot to the disk. So my concern would be if the FMM lock was placed on that drive, might it apply the PIN lock to your Mac if you booted to the drive?
I Googled around a little just now and can't find a definitive answer to this. But I'd say it is a legit concern.
The safe way would be to mount the drive externally then format the whole disk, but like you said, since nobody makes an enclosure to do this there is no way to format externally.