Im looking at this as well. I might be completely wrong but i think the licenses would be the issue....i think if you own the license then you cannot "lease" or "rent out" the license for the customer that resides on that single CUCM.
Isnt HCS licensing giving the provider the ability to now lease out which is why hcs licensing is different from lets say buying 20 UCL licenses. As for support, other than the licensing i dont really see an issue other than
1. Doing some type of third party non supported external directory to filter stuff
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.