A configuration engine such as LCFG could peacefully coexist and from the sounds of it even complement the functionality provided by oneSIS. Instead of modifying the configs of remote files, it would simply be modifying the different variations of a file in the master image (ie: /etc/fstab.myclass).
Yes this would be possible - LCFG doesn't care what the agents actually do. We have ones that run as pseudo components on "network controller hosts" that then talk to the switches/routers in whatever language they talk. VLAN/routing config on the switches changes based on the client host's profile (Yes it's scary). In theory you could also use it do things like create and start/stop UML images that in turn used LCFG/OneSiS inside the images.
We've had experimental versions running on Windows and OS X too.
Don't get me wrong, it's no silver bullet though. It has its flaws.
People interested in this stuff might want to take a look at this:
http://www.usenix.org/events/lisa04/
Carwyn
