7.9 Quoted line items - on-change controller

Has anyone figured out where to define on-change events for Quoted Line Items?
We need to look up pricing in a third party ERP based on mft_part_num. I already removed the populate related for the price field using the Extension framework, and now I'm thinking an on-change event on mft_part_number with an API call should do the trick.... but I can't seem to figure out which controller to extend.