OK, so whilst this can't be setup in a true HA fashion I did come up with a workaround. Not that pretty but for us its functional
In the end I setup a v.small alwayson setup with 2 servers, and configured a listener for them.
Then i used a PowerShell script to sync the CMS between the two servers on a schedule, using the listener as the source of truth (so which every server is the primary at the time).

I documented it here on my blog, using modified scripts from Chrissy LeMaire
http://mumblestiltskin.blogspot.com.au/2017/05/central-management-servers-ha.html

It does mean that the CMS is running if a particular server is down.

Regards