Can I define S4 accessor `@` custom for a class?

I'd like to define two S4 classes, where one contains the other. The lower class has a particular slot, say x, and I need object@x to behave differently depending on whether object is the lower class or the wrapper class. For example:

setClass("foo",
         contains = "numeric",
         slots = c(x = "character"))

setClass("bar",
         contains = "foo")

ff <- new("foo", 1:10, x = "abc")

bb <- new("bar", ff)

Now, from this I get:

> ff@x
[1] "abc"
> bb@x
[1] "abc"

What I'd like is when calling bb@x, to intercept the "abc" and modify it, such that bb@x produces something different than ff@x.

I tried setting a custom @ method for bar:

setMethod("@", signature(x = "bar"),
          function(x, slot) {
            out <- x@slot
            if (slot == "x") {
              out <- toupper(out)
            }
            out
          })

but it fails with this error:

> Error in setGeneric(f, where = where) : 
  must supply a function skeleton for ‘@’, explicitly or via an existing function

I tried using method.skeleton, but this errors as well:

> method.skeleton("@", "bar")
Error in genericForBasic(name) : 
  methods may not be defined for primitive function ‘@’ in this version of R

Any other options?