Hi FroggEye,
Running BE on a VM won't be an issue. My CASO server is a VM, and I don't have any issues yet (touch wood). That would basically be a means to an end in itself. As long as that BE VM can see the other servers, it should work.
The only issue you'd have here though is hardware. Unless you're load balancing between 2 hosts, how would you move your tape autoloader/drive/library if it's physically attached to 1 of the hosts? Here a SAN SSO setup would be great, as you'd present the SAN library to both hosts (across a virtual FC HBA), and it would move with the VM (thinking along the lines of our VMs in ESX here which work that way). If you've got 1 host, it will also work, but not with 2. This is where running BE as a VM would fail.
Disk speed for the BE VM will also be slower due to sharing the same disk spindles with other VMs.
As far as I know, GRT would work with just the agent being installed. That's how Exchange works at least.
As for backing up the VM itself, you'd run it exactly the same way. Backup your necessary/required folders and files.
I'll continue to dig around.