Setting aside the "why?" for a moment, would it be possible to install a Xen hypervisor and have a Solaris DomU with several Zones / Containers running?
Just one of those hypothetical questions that occurs at random times, not actually thinking of trying it out just yet.
I see no reason why not, as long as you can get Xen to host Solaris -- I use this exact setup with several different versions of VMware with no issues.
In my environment, I have Sol10 VM's with anywhere from 1 to 30 zones running on top of ESX, ESXi or Fusion. The zones live on a secondary (or more) disks, either as virutalised disks or raw LUN's presented from a SAN, formed into a zpool.