Thanks! I'll suggest to fleet mgmt they contact Heathrow twr with a view to changing the notice.
FWIW I believed that 777, 744s were being (or have been) modded to remove the Nose gear compression inhibit from the txpdr. I believe that BA's 767s have already been modded, but I can't remember why the mod was done. (I don't think it was connected with Mode S gnd use).
As for other a/fields, I can only say that JFK haven't yet complained about the Mode A/C clutter, but I guess that 99.9% of their tfc doesn't squawk til line-up.
Thanks again, folks.