The ballhead is placed onto the unit with a 1/4"-20 screw. The head of the screw has a D-ring that folds flat to make it flush with the bottom of the TrailPix unit. (A common type of screw in photography.) When you "open" that screw up, the D-ring is large enough to fasten some light rope to, so you can indeed hang some weight. I hung 40 lbs. from the universal made with THREE trekking poles, and the screw seemed fine (The manufacturer doesn't rate it for this application, I asked).
However, I would think 20 lbs. would be reasonably safe. I would limit the load to 10 - 20 lbs. for either the custom or the universal when made with the accessory pole as the pole is optimized for it's own weight, not stiffness.
Having said all that, I can't guarantee miracles for TrailPix in heavy winds. The torsional stiffness is not fantastic, a by-product of the way everything is hooked together. I find it perfectly usable, but someone used to a high-end tripod is going to notice a significant difference in stiffness.
Ultimately, a judgment call.