I wonder if it is really not possible. You might need to do the installation totally manually.
I've done such manual installation for a customer who had a very specific internal WAS farm. And they didn't want to use the IBM installer and deployment.
The only problem I will see with Tomcat instead of WAS, is all the authentication method that are integrated with WAS and not with Tomcat, and that would be the problem I would see in order to use the current ICN with Tomcat.
So for me, I would say... ME TOO !! :-)