Iodine in alcohol or xylene is reputed to remove mercury deposits, 
but as Peggy Wenk has pointed out, it creates an interesting disposal 
problem.  Your hazardous waste hauler must be informed that mercury 
is present.  They can then advise you if it is feasible to use such a 
de-Zenkerizing solution.

Keep this in mind, whether you use a water-based or solvent-based 
system to remove mercury deposits:  that solution plus at least the 
next one will contain mercury.  Waste with as little as 1 part per 
billion is prohibited from going down the drain in the US.  In the 
EPA Region I (New England), there is a mandate to virtually eliminate 
ANY mercury from wastewater by 2005.  Current technology allows 
detection down to 0.1 ppb, so that will be the new level by 2005 if 
finer detection limits are not available by then.

This points up the futility of using any mercury-containing fixative. 
You cannot operate your lab and be in compliance with US 
environmental regulations.

Zinc does not leave a pigmented deposit and thus does not need a 
de-Zenkerization step in staining.

Dick