Copy a file's NSURL to clipboard
Here is the solution to the issue (from: http://macscripter.net/viewtopic.php?pid=183164#p183164):

use framework "Foundation"

on run {input, parameters}

    set p to POSIX path of (input)
    set nsu to current application's |NSURL|'s fileURLWithPath:p

    set fru to nsu's fileReferenceURL()'s absoluteString() as text

    return fru

end run


I did not check jweak's answer. It may also work, after all.
The poster over at macscripter also mentioned something important: input may contain a list of items (multi-selected).

Hope this helps someone.