So, I've been trying to figure this out, but have not made much progress. I turned off all DLNA sharing stuff on the HTPC (which had been showing zones from the laptop), and turned all DLNA sharing off on the laptop also. I then restarted both machines, then turned on sharing for all services on the HTPC. I turned on sharing on the laptop and 2 of the 3 zones from the laptop immediately showed up on the HTPC. I don't know why the 3rd zone isn't showing up, but it is the only zone which is set for HDMI playback (the other 2 are set for Direct Zone and the HD Audio Headphone jack).
So, I'm now thinking that HDMI zones cannot be shared or controlled, but that seems 'wrong' to not allow an HDMI zone to be controlled by another controller.
I have all DLNA sharing turned on for both computers, and only 2 of the 3 zones from the laptop show on the HTPC, and the only zone on the HTPC does not show on the laptop.