How to perform a check on a permutation "on-the-fly" without storing the result in R

Assume we have the following permutations of the letters, "a", "b", and "c":

library(combinat)
do.call(rbind, permn(letters[1:3]))
#      [,1] [,2] [,3]
# [1,] "a""b""c" 
# [2,] "a""c""b" 
# [3,] "c""a""b" 
# [4,] "c""b""a" 
# [5,] "b""c""a" 
# [6,] "b""a""c"

Is it possible to perform some function on a given permutation "on-the-fly" (i.e., a particular row) without storing the result?

That is, if the row == "a""c""b" or row == "b""c""a", do not store the result. The desired result in this case would be:

#      [,1] [,2] [,3]
# [1,] "a""b""c" 
# [2,] "c""a""b" 
# [3,] "c""b""a" 
# [4,] "b""a""c"

I know I can apply a function to all the permutations on the fly within combinat::permn with the fun argument such as:

permn(letters[1:3], fun = function(x) {
  res <- paste0(x, collapse = "")
  if (res == "acb" | res == "bca") {
    return(NA)
  } else {
    return(res)
  }
})

But this stills stores an NA and the returned list has 6 elements instead of the desired 4 elements:

# [[1]]
# [1] "abc"
# 
# [[2]]
# [1] NA
# 
# [[3]]
# [1] "cab"
# 
# [[4]]
# [1] "cba"
# 
# [[5]]
# [1] NA
# 
# [[6]]
# [1] "bac"

Note, I am not interested in subsequently removing the NA values; I am specifically interested in not appending to the result list "on-the-fly" for a given permutation.