Opening raw disk while mounted in 5.x?

[David Andersen]

Hoping someone knows the quick answer to this - in 4.x, it was possible 
to open /dev/ad0 while a filesystem one one of its slices was mounted.  
This no longer appears possible under 5.x.  Could someone point me to 
the spot in the code where I'd need to disable a permissions check (or 
a sysctl, or anything) to permit this behavior again?

(The context:  mounting slice 2 of a disk and using it to store a 
compressed filesystem image.  Then opening the primary disk device to 
directly write the filesystem image onto slice 1.  The kernel seems to 
muck with the write calls if we try to do the write onto slice 1 
instead of the raw disk).  This is using the very cool 
imagezip/imageunzip utilities from the Utah Emulab project.

Thanks!

   -Dave