Only a batch file (echo text >com1) or is there also a .exe with the displays? First case should work in windows 2000, if not try to set the handshake of that port to none (device manager -> your com port -> settings). If there is an exe it might work with a Dosbox CVS.
In the dosbox.conf:

[serial]
serial1 directserial realport:com1
# serial1 is the com1 inside dosbox, com1 is the Windows port name

This is the exact line copied from my shortcut:
C:\walld\NY\send.bat C:\walld\MOC\opscon0 1

Here is the contents of my BATCH file
@ECHO OFF

REM CE FICHIER DE COMMANDE PERMET DE COMPILER, LINKER, ET ENVOYER
REM DES MESSAGES DEJA EDITES A UN PANNEAU VIPCO NEW YORK.

IF EXIST %1.MSG GOTO CPL
ECHO.
CLS
ECHO Voici la liste des messages disponibles:
DIR /W/ON *.MSG
ECHO.
ECHO Veuillez ecrire SEND, le nom du fichier sans extension et
ECHO le num‚ro du panneau
ECHO.
GOTO END
:CPL

COMPILE %1.MSG %1.DAT
LINKADD SEND.ADD %2 %1.DAT
LINKLOC SEND.LOC 663-8552 9600 SEND.ADD
CALLX LD SEND.LOC
DEL *.LOG
ECHO.
:END

Here is the dosbox.conf file
directserial=true
comport=1
realport=COM1
defaultbps=9600
parity=N
bytesize=8
stopbit=1

And I do not understand what you mean by the "serial1"

Hmm more apps involved.

1[autoexec]
2mount c c:\
3C:\walld\NY\send.bat C:\walld\MOC\opscon0 1

> And I do not understand what you mean by the "serial1"

Old dosbox 0.63 uses

1[directserial]
2directserial=true
3comport=1
4realport=COM1
5defaultbps=9600
6parity=N
7bytesize=8
8stopbit=1 

Newer Dosbox CVS builds use

1[serial]
2serial1 directserial realport:com1

It works !

Thanks a bunch