It is an explicit hack, as I describe below on variants_including_master_controller.rb.
This is actually based on the backorderable checkbox implementation. However, Variants are managed through the ResourceController, which uses heavy convention to make it's updates to the model. If I use the checkbox exclusively, then no value is passed when the checkbox is unchecked and, as a result, the ResourceController does not update the model. This obviously will not work. 

It is the "rails-y" aspect of the Variant/Resource controller that forced me to use this hack. The only other option I could think of is to have a separate controller with it's own explicit code just to set the track_inventory flag, but that seems like an equal or worse hack, especially considering it would duplicate code and potentially introduce bugs or security vulnerabilities (the ResourceController already manages authorization for the action). Any suggestions on alternative implementation are welcome.