Thank you, Steven. I've added Feature Request to the tags. I assumed I might just be missing something.
A little research turned up the fact that there's apparently an issue with system latency involved in this. If I turn off latency reduction, then, unless I have "Tape style" monitoring turned on, A track will be audible even if Monitoring is turned on. In other words, if I turn on Low Latency Monitoring in the Main Out channel, then input monitoring repalces track audio. If Low Latency Monitoring is off, then both the track and the input are monitored. It looks as if reconciliation of this isn't easy, but if it can be done using the per-track record button (with Low Latency Monitoring enabled) then surely it can be done if the transport record button is pressed to turn on recording in armed tracks.