During some rotuine testing here at OSR, it became apparent to us that the pool tags associated with allocations that came from Special Pool were not being tracked. So, we did some checking.
It seems that in both Windows XP (at least through SP1a) and Windows Server 2003 when a driver is run under driver verifier using the "special pool" option, the driver's allocations that come from special pool are not tracked by pool tag.
Sources at Microsoft say that not tracking pool tags for special pool allocations was intentional. Whether this decision will be changed in future Windows releases or service packs is not known at this time.