Hi Ty,
The "skullcaps" that I've seen on the IVO props are relatively small diameter, attached with a single screw through the center. The small diameter equals a shorter moment arm for the weights attached to the inside surface of the spinner. So it would take more weight to get the same effect than if using weights attached to the inside of the backing plate flange on a "standard" spinner design. That flange is the usual and very convenient location for mounting balance weights. Trying to install reliable weights to the inside of a skullcap would be a difficult trick. On a Lyc engine with a large starter ring gear, weights can frequently be easily added to that flywheel. It's usually not desireable or convenient to add or subtract weight directly to/from a propeller in the field.