I don't know about moving the zRivet... but what you could do is:
select the zRivet in the outliner, and then Maya Top Toolbar > Ziva > Remove > Rivet to Bone (make sure to do it this way because it'll remove any connected nodes as well)
then right-click-hold on the curve and select "control vertex"
select the control vertex you want to be zRiveted, and move it in proximity to the location of the bone that you want.
then shift-select the bone, and Maya Top Toolbar > Ziva > Add Tissue Property > Rivet to Bone
updateToPost for other people who may find this useful:
Straight from the source: "To modify the rivet node in any way, delete the node and create a new one."