Yes! I managed to get RouterOS 6.6 to run on XenServer 6.2. The VM in question was running 5.26.
With XenServer 6.2 (and perhaps earlier) they added support for Windows 8.1, and this support is confusing the mikrotik upgrade/install.
First you'll need a console to the Xenserver Host, so you can change the VM parameters. SSH or XenCenter should work fine.
Secondly you need to find the UUID of the VM in question. Use "xe vm-list" to show all the VMs on the host. Look for the Mikrotik VM, and copy the UUID (The clipboard is your friend here..)
Last step is to turn off the "viridian" support..
Using the uuid in the first command, enter "xe vm-param-set uuid=<UUID from first command> platform:viridian=false"
This turns off the viridian support for the VM.
You can now start a new VM for install, or upgrade an existing one. If you had a VM that was crashing during the 2nd stage of the upgrade, this should allow it to continue.
In my testing, the VM Upgrade that had crashed, worked, but locked up on "starting Services...". Forcing a reboot allowed the VM to function normally.
A 5.26 machine was changed, then upgraded, it functioned normally except it failed to reboot on the first stage when it should have. Forcing the reboot allowed it to continue fine, even through the 2nd stage.. go figure!
A fresh install using a 6.6 ISO failed again to restart after install, but a forced reboot had it boot up to login first time.
I suspect disabling ACPI or other flags may resolve the reboot issue, but at least it isn't crashing out now!