[Nonfractal]

I'm afraid that I don't know of any way to balance an armature without opening the can.
You have got me thinking about how it might be possible though. 
Alan.

epoxy putty shoved into an armature stack gap, just like NSR do...maybe the green epoxy putty normally found has come off that motor, there may be evidence...

True Kev,
However, the problem isn't fitting the putty , resin or other substance to the armature ,
 It's figuring out where to put that weight. 
Maybe trial and error would work but something that could be added and removed would probably get spat off during testing.
Of the 5 shark motors I have lying around in boxes, only one shows any evidence of balancing (see blue resin in the pic) 

Alan