I have not done it, but here is a reply from another forum answering that question:
You need to create a demoloop.cfg file and put the following inside of it:
set s_initsound 0
snd_restart
timedemo 1
set loop1 "demo demo001.dm3;wait;set nextdemo vstr loop2"
set loop2 "demo demo002.dm3;wait;set nextdemo vstr loop1"
vstr loop1
this will create a loop of the two demos that come with the game. You can add more demos somthing like this:
set s_initsound 0
snd_restart
timedemo 1
set loop1 "demo stadfog.dm3;wait;set nextdemo vstr loop2"
set loop2 "demo pumped.dm3;wait;set nextdemo vstr loop3"
set loop3 "demo quaver.dm3;wait;set nextdemo vstr loop4"
set loop4 "demo killwig.dm3;wait;set nextdemo vstr loop5"
set loop5 "demo crusher.dm3;wait;set nextdemo vstr loop6"
set loop6 "demo demo001.dm3;wait;set nextdemo vstr loop7"
set loop7 "demo demo002.dm3;wait;set nextdemo vstr loop1"
vstr loop1
Once this file is created, all you do run the quake3.exe with the following after it +exec demoloop.cfg. You can create a batch file to do it like this:
@echo off
d:
cd\games\QuakeI~1
quake3.exe +exec franklocal.cfg +exec demoloop.cfg