macro pushx [arg] {
common
  local flag, cnt
  cnt = 0
  flag = 1
forward
  local sz
  cnt = cnt + 1
  if arg eqtype ''
    virtual at 0
      db arg
      sz = $
    end virtual
    if (sz > 4) | (cnt>1)
      flag = 0
    end if
  end if
common
  local lbl

  if flag
    pushd arg
  else
    pushd lbl
    lbl text arg
  end if
}    

match x, `arg {
  match x, arg \{
    flg equ TRUE
  \}
}    

macro tellquoted param
{ 
  match `param,param \{ display "Yes, it's a quoted string: ",param,13,10 \}
}

tellquoted 'ABCD'
tellquoted abcd
tellquoted 1234    

stdcall MyProc, <"Hello", 13, 10, "World!">    

stdcall MyProc, '1234'
; will generate:
push '1234'
; instead of:
push lbl?
lbl? text '1234'    

stdcall MyProc, <'123','4'>    

the exact syntax of the latest macros allows all of described options.

What kind of control you need on the terminating character?

stdcall MyProc, <"Hello", 13, 10, "World!", 0>    

Btw. Do you also allow to pass one string in Unicode and one string in ASCII?

I know FASM have "du" directive, but you can define text using only ASCII text - i.e. less than 256 symbols.

invoke MessageBox,NULL,<ustring "Hello, world!",0>,<ustring "Title",0>,MB_OK    

But you can't use "Cyrillic" and "Japan" in the same time, while using UTF-8, it is possible.

invoke MessageBoxW ,NULL,"Hello, world!","Title",MB_OK    

stdcall ShowMessage, [parent], smiInformation, 'Title', 'Message text.', smbOK    

The above procedure needs UTF-8 encoding and will be compiled properly for Windows and Linux with every language you want.

Would it be possible to do the same using the portable features of FreshLib only?