Would not the simplest solution in HKG be to only line up airplanes on 7R from J and H when using the runway in mixed mode? Just make all GA and Cargo aircraft cross at J6 and join the cue on the north side of the runway. By not using K1 you would avoid any issues.
Or just hold aircraft at the Cat 2 hold. I don’t understand why that is so hard. If you time it right then it makes absolutely no operational impact. If the planes are that tightly spaced, holding at the CAT 2 would not make that much difference to holding at K1.