Big to little endian just swaps the bytes around in pairs of twos...

Code:
alias blendian {
  var %i = 2, %result
  while ($mid($1,%i,2) != $null) {
    %result = $+($v1,%result)
    inc %i 2
  }
  return %result
}


$blendian(1234) = 3412

It's not an "expert algorithm" - 99% of your problems can be solved with simple string manipulation.