Lori nicely explained how a BSgenome can be converted into a GenomicRanges::GRanges object.

Would it be useful to turn this into an exported S4 method in GenomicRanges or BSgenome?

methods::setAs( "BSgenome", 
                "GRanges", 
                function(from)  as(GenomeInfoDb::seqinfo(from), 'GRanges')

It depends on how obvious the conversion is. Sometimes, probably including this time, it's best to be explicit. Others may have a different opinion though.