In a running project I found it very convenient to have a local getSeq() method for GRanges, especially for interactive use.

getSeq_GRanges <- function(x, ...) {
  genome <- unique(genome(x))
  if (any(is.na(genome)))
    stop("Can not getSeq objets where `genome` is not defined.")
  # Maybe should also check that length(genome) == 1
  getSeq(getBSgenome(genome), x)
}
setMethod("getSeq", "GRanges", getSeq_GRanges)

Would you consider adding such a method to the BSgenome packages or wherever it makes sense ?

Have a nice day,

-- Charles