Bulk import of custom attributes

Support suggested to push a txt file via policy named as some grouping information (such as Student.txt or Staff.txt or sub-group) and then run a smart group based on that file, on each individual machine.

I still wonder if there's any way to access MySQL and modify the table directly.

ANY HELP ?!?!?!

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I would have had the same suggestion. Create either a dummy .pkg install (no payload) or drop some file on the systems that will help you gather those Macs later by either looking for the presence of the receipt, or using an EA to look for the file.
Since you stated you already have most of the Macs in a Static group now it shouldn't be hard to target the correct Macs for deployment.
Going forward you could modify your imaging workflow by creating a configuration for each type (if you don't already do this) and throw the appropriate package install into each one to help identify them out of the gate. That's basically what we do to identify Macs for each company under the larger umbrella.

Edit: There is one other option. Provided its not being auto populated, and thus overwritten by your LDAP environment, you could use something like the "Department" field in the Location tab to tag the Macs accordingly. Just because its labeled as "Department" doesn't mean it has to be a department per se. It could be a group of Macs or users of those Macs, like Students, Staff, etc.
If you have some way to pull up all Macs that are part of the same group in an Advanced Search, you could update the Department field for all of them in one shot with the Take Action on Results pull down menu. Sadly you can't do this with the "Room" field, which might be more appropriate in some cases. For this to work and stick, you'd need to make sure the "Department" field isn't going to get updated from LDAP during an inventory report for example, so you may have to adjust your mappings.