Could you not scope secondary instead by using a couple of extension ht leads.

sounds similar to the later rover k series setup. At least you could scope 2 of the secondary ht outputs but I suppose its going to be the coil on plug cylinder at fault.

Is it not possible to scope current on these 2 ht output coils?

I've never really had much luck scoping ignition systems to find intermittent misfires unless its really severe then I can notice a fault.