Or to enable some language other than SQL (QUEL anyone?)
A few years ago, I got a minimal POSTQUEL working again to release as a patch for April Fools' Day, which I never did. I should dig that up somewhere :)
Anyway, as far as OP's original question regarding replacing the grammar, there are a couple of such implementations floating around that have done that. But, I actually think the pluggable parser patches were good examples of how to integrate a replacement parser that generates the expected parse tree nodes for anyone who wants to do their own custom parser. See Julien Rouhaud's SQLOL in the "Hook for extensible parsing" thread and Jim Mlodgenski's "Parser Hook" thread.
