create a new file in baseq3 and call it demoloop.cfg (or whatever you want to call it)
in that file put:
timedemo 1
set loop1 "demo demo001;wait;set nextdemo vstr loop2"
set loop2 "demo demo001;wait;set nextdemo vstr loop1"
vstr loop1
save the file.
now when you enter the game, bring down the console and execute the file:
/exec demoloop
hit escape when you want to stop it.
-patchy