setClass("MyDb", representation(data = "data.frame", version = "character"))

setMethod("$", signature = "MyDb",
          function(x, name) { return(x@data[[name]]) }
)

db <- new("MyDb", data = mtcars, version = "2.3.1")

library(sp)
data("meuse")
coordinates(meuse) <- ~ x + y
meuse$  # brings up autocomplete options for meuse@data slot

# in MyDb.R
names.MyDb <- function(x) names(x@data)

# in NAMESPACE
S3method(names,MyDb)