Dear Andrea,
I had a very similar problem, and the solution that was suggested to me is pretty much what you already do. The most notable difference is the suggestion to check for the kill variable only where necessary (if the geometry and the logic of the problem allow it) to save some CPU time.
This can e.g. be done by creating copies for the materials surrounding the trigger volume and activating usrmed for those materials only.
Cheers,
Roman