More on this, was at the client site yesterday. Note that I've now tried two completely different control systems (MX880z + MRZ260, and a MX900i + MRF260i), and the issue is the same.

What I've learned is that the only thing works is having the remote directly emit IR. Using RF, and then having the MRZ or MRF base station emit IR (either via a flasher, or via a 3.5mm mono patch cord to the Toshiba's rear IR jack) does not work. The flasher illuminates, but regardless of where on the TV the flasher is placed (obviously, including where the manual says the sensor is located), there is no response from the TV.

If the remote is used as "IR Only", everything is fine, including the ability to use previous IR command sets from the URC database, and the ability to learn IR from the original remote. When used in RF mode, nothing works - not the original codes nor learned codes.

It sure sounds like something basic - but I've tried just about everything - different ports on the base station, different cables, different IR flashers, etc. Quadruple checked my IR routing. But nothing works on this TV except having the remote emit IR directly.