It's very difficult to troubleshoot a problem that is not in the logbook. It's also usually frowned upon when you start taking apart serviceable airplanes in scheduled service.
If these pilots were so concerned about the previous rad alt problems, they should have documented the faults in the logbook. At that point, it would require someone to take some action that would have to be documented. Everything would be very clear then.
It's actually amazing how much troubleshooting on rad alts can be done with the aircraft on the ground.
I would personally prefer that pilots not troubleshoot faults during the approach. It's obviously a critical time which requires the pilots undivided attention. Systems like A/Ps and A/Ts only require monitoring during the approach phase. There are switches on the MCP that turn off systems that are not functioning correctly. No need to search for circuit breakers. After the aircraft lands, any faults can be written up in the logbook.