Another method you could try is using the ladder script that Xplosys made for his ladder entity.
;Artificial Intelligence Script
;by xplosys
;Header
desc = Ladder (Automatic in range)
;Triggers
:state=0:state=6,coloff,setalphafade=0
:state=1,raycastup=60 80:state=5,unassociateplayer
:state=1:associateplayer,moveup=2
:state=5,raycastup=0 -20:state=6
:state=5:moveup=-2
:state=6,plrdistwithin=40:state=1,associateplayer
;End of Script
The way it works is you place the ladder in position and make it dynamic, and assign the above script to it. Then we place a second ladder over top and make it static, and leave it's ai script as default. I've used it and it works fine.
In your case, just don't use the ladder or make the ladder static or try to assign the script to a trigger as suggested. Playing around with this may give you some ideas.
Mike