Actually the Grouping Tool lets you select groups based on Material Groups. And most characters have the inner mouth, teeth, tongue and lips as separate materials already. Simply select all other materials with the Grouping tool, and assing a new material ("Invisible would be a good name" ;) )

and you'll be left with a mimic-compatible mouth.

Like this:

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.