I see what your meaning with your diagram and only extending the chainstays will make the force on the rear wheel less ie easier to skid but I still think a track frame is easier to skid. Your weight at the front is acting over the front axle so activly trying to pivot the rear wheel up round the front axle instead of acting behind the axle and still pushing the rear wheel down slightly, I think this would have more of an effect. I also rode a conversion that I could never skid well but found it easy using my brothers track bike, then when I got my own track bike that was fine too but I still cannot skid the conversion anywhere as easily as either of these track bikes.
I see what your meaning with your diagram and only extending the chainstays will make the force on the rear wheel less ie easier to skid but I still think a track frame is easier to skid. Your weight at the front is acting over the front axle so activly trying to pivot the rear wheel up round the front axle instead of acting behind the axle and still pushing the rear wheel down slightly, I think this would have more of an effect. I also rode a conversion that I could never skid well but found it easy using my brothers track bike, then when I got my own track bike that was fine too but I still cannot skid the conversion anywhere as easily as either of these track bikes.