Another quick update, I haven't made much progress this week on rigging because I had a hell of a time getting all of Merlin's accouterments rendering correctly in Maya with Mental Ray.  I kept pushing all the geometry into Maya via GoZ and the displacement maps were waaaay off.  More on this after some pictures!

Below are a few images, in varying degrees of zoomed-in-ness, showcasing Merlin in his t-pose rendered from Maya using Mental Ray.

Slick stares joyously, or demonically, at the wee candy man.
Merlin tries very hard not to move - must not get eaten by giant creepy man!
A look of sheer terror - or perhaps a blank look before facial rigging... you decide.
For those that are interested in why the displacement maps got out of whack: I deduced through a bit of online research and troubleshooting that the problem boiled down to their Alpha Gain and Alpha Offset.   It appears the Alpha Gain and Alpha Offset coming from ZBrush work best under centimeter scene scale - I was using feet - however adjusting them can get pretty good results too.
The displacement map's Alpha Gain and Alpha Offset in the Attribute Editor
To adjust the Alpha Gain and Alpha Offset to match scene scale I tried what seemed like a logical scale difference - multiplying both numbers by 30.2 (which is the number of centimeters in a foot).   However, this yielded freakishly large ballooned geometry.   A bit of trial and error (trying different multiplication factors) and I got something that was displacing pretty close to the original ZBrush model.

UPDATE: After completing the above ranting explanation of Alpha Gain and Alpha Offset I discovered 32-bit displacement maps from ZBrush would have eliminated this scale issue completely, doh!   Now I know, and knowing is the battle...