Boundary constraints

I have to perform static condensation on large matrices. What this means to Metis is that I need to constrain a group of "nodes" such that they remain in a block at the bottom during the resequencing of the unconstrained nodes. I've faked it by ignoring the constrained nodes, resequencing the unconstrained nodes, and then adding the constrained nodes back into the connectivity. This works, but if there are a LOT of unconstrained nodes, the fill becomes substantial. Is there a way to trick Metis into leaving a group of nodes at the bottom where they originally appear in the sequence, yet considering the consequences of their connectivity on the fill while sequencing the others?