I'd really like the answer to be yes on this one!
Looking at the specsheet for this device, it is ideal for our needs, with one crucial missing piece : H323 Gatekeeper.
I was thinking that we could run a virtual session for this one system, and run the rest from RouterOS on the one board, but this is only possible AFAIK with the RB400 series, and these only have support for MicroSD which is unsuited to cacheing.
If the PPC-based Routerboards had access to virtualisation they would be a perfect match for our requirements.