Override [.data.frame to drop unused factor levels by default

Question

The issue of dropping unused factor levels when subsetting has come up before. Common solutions include using character vectors where possible by declaring

options(stringsAsFactors = FALSE)

Sometimes, though, ordered factors are necessary for plotting, in which case we can use convenience functions like droplevels to create a wrapper for subset:

subsetDrop <- function(...){droplevels(subset(...))}

I realize that subsetDrop mostly solves this problem, but there are some situations where subsetting via [ is more convenient (and less typing!).

My question is how much further, for the sake of convenience, can we push this to be the 'default' behavior of R by overriding [ for data frames to automatically drop factor levels. For instance, the Hmisc package contains dropUnusedLevels which overrides [.factor for subsetting a single factor (which is no longer necessary, since the default [.factor appears to have a drop argument for dropping unused levels). I'm looking for a similar solution that would allow me to subset data frames using [ but automatically dropping unused factor levels (and of course preserving order in the case of ordered factors).