Yeah that would be a pain to track, you could use mag/slot numbers as any replacements should be the same size and will go in the same slot or depending on the models the rpm value might be 100/150 (but I think only the smaller drives came at different speeds). Probably the best option would be the number of chunklet filters;
Code:
-tc_gt <number>
Specifies that physical disks with total chunklets greater than the number specified be
selected.
• -tc_lt <number>
Specifies that physical disks with total chunklets less than the number specified be selected.
But this isn't something I've needed/wanted to do so all theoretical.