I'd say that there is no surefire way to get the lil buggers to go when
you want them to.  I think ferrets generally go within a short time after
waking up, about five or ten minutes.  I try to wake mine up when its
playtime and keep them in the cage for a while longer, hoping that they
will go then.
 
As far as expunging (?) the bladder..I've seen it done once, when the vet
I volunteer with was going to spay a cat.  For one thing, the cat was
anesthetized already, and on its back.  The tech pushed gently on the lower
abdomen, and moving towards the rear, and sure enough, the cat peed. ;)  If
the vet needs a urine sample, or for the ferret to go, it would probably be
better to let them deal with it.  Maybe put a litter box or newspaper in
the cage on the way to the vet.
 
Hope this helps!
brenda
[Posted in FML issue 2768]