Does anyone have BNF notation for the atom specification 'language' used
by Chimera? 

I don't think so.  What's the use case here?  If you're in the context of Chimera, you should be able to use Chimera's own atom-spec parsing facilities, e.g. chimera.specifier.evalSpec().  Outside of Chimera, I'm aware of a pretty extensive but not totally complete parser in perl:  Chemistry::MidasPattern - Select atoms in macromolecules - search.cpan.org

--Eric