dispatching S4 methods with an expression as argument

I'm trying to convince an S4 method to use an expression as an argument, but I always get an error returned. A trivial example that illustrates a bit what I'm trying to do here :

setGeneric('myfun',function(x,y)standardGeneric('myfun'))

setMethod('myfun',c('data.frame','expression'),
          function(x,y) transform(x,y) )

If I now try :

> myfun(iris,NewVar=Petal.Width*Petal.Length)
Error in myfun(iris, NewVar = Petal.Width * Petal.Length) : 
  unused argument(s) (NewVar = Petal.Width * Petal.Length)

> myfun(iris,{NewVar=Petal.Width*Petal.Length})
Error in myfun(iris, list(NewVar = Petal.Width * Petal.Length)) : 
 error in evaluating the argument 'y' in selecting a method for 
 function 'myfun': Error: object 'Petal.Width' not found

It seems the arguments are evaluated in the generic already if I understand it right. So passing expressions down to methods seems at least tricky. Is there a possibility to use S4 dispatching methods using expressions?

edit : changed to transform, as it is a better example.