While the tracks are recording it is impossible to unarm them. We've already come to that conclusion. BUT - what you CAN do is while the transport is running, use Record Toggle to switch to playback mode. In this state you CAN arm/unarm the tracks while the transport continus to run.
So, in effect, you can sort of get what the original poster wants. It's just that it takes a few steps rather than one. You need to toggle in/out of record mode - then make your switch as to the record armed state of the track - then toggle back to record.
So.... there you go