There is a BNF grammar for the built-in Inventor nodes and fields at
ftp://ftp.sgi.com/sgi/inventor/2.0/InventorBNF.ps.Z
It doesn't include WWWInline/WWWAnchor, includes a lot MORE than VRML, and
doesn't include user-defined nodes.
YACC can't handle all of Inventor because the extensibility features make the
grammar dynamically extensible (are there any parser-generators out there
that allow self-describing data?).