Hi there,
Sorry for the slow response but have been focused elsewhere.
You can actually use vmkping on the iSCSI interface by using the -I option to specify the vmk to use. Here is an article that explains it better than I can: https://kb.vmware.com/s/article/1003728
One thing to note - make sure you use the -d option so that packets cannot be fragmented to fully test.
Let us know how this works out.