You should see a big difference with HF. Remember to point the counterpoise toward your distant station.
You'd probably still see improvement if you could somehow get it around the front BNC, because electrically the antenna will be the rubber duck plus the rat tail. Putting it on the ground lug makes the antenna the rubber duck, radio chassis and then the rat tail. MFJ 18xx antenna literature quotes different math for making ground lug counterpoises, for instance. Still. I wonder if it'd help on 6m..
I would really hate to see the size of "mousie" that would have "mousie tails" suited for an AM broadcast tower installation... Dave W7UUU