Thanks for the answer, I also came to the same conclusion after posting. In case it helps others below is what I learned.
1) The quota modify api calls are listed under the "Vserver" section in zexplore, not the "Cluster-Mode" section.
2) To use the Vserver section of calls you need to target a mgmt enabled lif on the SVM and not the cluster mgmt lif.
Sample code can be seen in zexplore (in my case python) and the IP or Name in the NaServer object needs to me your SVM not the cluster SVM. To modify what I wanted (just the disk space allowed)
my object looked like this:
api = NaElement("quota-add-entry")
api.child_add_string("disk-limit","10G")
api.child_add_string("policy","default_user_quota")
api.child_add_string("qtree","")
api.child_add_string("quota-type","user")
api.child_add_string("quota-target","7003")
api.child_add_string("volume","user_vol18")