Thanks everybody for your input.  Let me throw out some more information and
see what you think.

The current process is direct die attach to a heatsink through a cutout in the
PCB.  We want to save some process steps without losing the thermal
dissipation.  So, we looked at thermal vias.  But the problem we ran into was
amount of metal to pull the heat away and the requirement to attach the
heatsink first.  By attaching the heatsink first and applying conductive epoxy
for die mount, the entrapped air blows holes in the epoxy and displaces the
die.  We didn't want to tent the die side with soldermask since we also need
electrical contact.  That's when we looked at some sort of fill method.

Anyway, more info to think about, but you all have provided enough info to
keep me going for a while.  Thanks again.

Glenn Pelkey
Quality/Reliability Engineer
Maxtek Components Corp.