Is it possible to configure intercluster trunks by way of dial peers via the CLI on the UC5xx to scale above the 5-site maximum allowed by using the Multisite Manager?
The CCA MSM is enforcing a hard limit of the system. It is not recommended to work around that and not supported.
Intercluster? Not sure what you mean. If you mean dial-peers for multisite - technically yes, but forget about CCA... you will need CLI.
I'm up to 6 sites with a client so far without issue. I think the 5-site maximum is more of a notional number - and certainly the limit of CCA.
As far as I am aware, there is no hard line CME limitation - you might just run out of DSP resources before you reach it. Cisco staff may be able to clarify.