Why not drill a hole through the bottom clamp and put a pinning bolt in? If it is below the standard mounting brackets I don't see it weakening the mast and it won't rotate.
But I don't understand "lever" and "leaning." What am I missing here?
Look around at commercial antennas that are bottom mounted with clamps to legs. Many of them get the "gangster lean" instead of standing vertical.
This is because each normal universal clamp only puts pressure on four points of the tower leg and four points of the antenna, instead of fully round compression. The leverage of the long antenna working on the clamp causes the clamps to slip and rotate.
If you understand bolts and fasteners and how they stay tight, the universal clamp it is a terrible system for resisting rotational forces caused by that long lever when the wind is in line with the clamp brackets (at right angles to the bolts).