Allowing access to array members (cache included) while the array is stopped is probably not going to happen, for various reasons Tom has stated before.
However...
I see no real reason that we couldn't allow VM's that aren't on the array to operate. It's not like you couldn't set up a VM configuration where all the required elements were on a non array device, but it wouldn't be able to easily share those elements with the VM's currently configured. You would need a separate XML storage location, and an iso location for install media.
Maybe a GUI section for array limited VM's, and another for VM's on unassigned devices?
As soon as limetech actually takes control of UD instead of relying on dlandon to maintain it, I see no reason why we can't have 2 VM types. However, as long as limetech doesn't actively manage unassigned devices, it's probably not going to happen unless someone writes a plugin.