150k to the 2N2222 base (mine's a 2SC945, again from that PSU) from +12V works to mute the audio. That, of course, is with the base otherwise open (and collecting RF!). i'll try 10k base to emitter ... after tea :)
Meanwhile I note quite a bit of audio hiss - worsening with proximity to the board, and particularly bad by touching the filter xtal cans. Can they be earthed without altering the filter too much? Might give better RF immunity if, like me, your unit is 'al fresco' or in a non-shielding enclosure.
