Pardon my ignorance, but it seems that in Ubercart 3, hook_order doesn't exist. hook_uc_order does, and by changing the name of the function to uc_pma_uc_order, the code runs as intended in the uc_order hook.
Closed: duplicate
1.0
Code
Closing this, as I addressed the hook_order in another issue and added a patch there.