Another question - does ISPConfig support running the interface on one machine, and actually performing the config work on another (or others)? Or is that something left to the backend plugins to implement?
ISPConfig 3 supports multi server setups with one interface node and as many slave nodes as you need. On the slave nodes you just have to install and run the "server" part of ispconfig (all scripts and files located in the server directory). The data is replicated by ispconfig internally, so the plugins have to do only the configuration on the local node were they are run. A requirement for ispconfig is that you have a mysql server running on every node incl. the master node. But as mysql is available for windows, this should not be a problem.
I guess that even mixed setups with some slaves on windows and other slaves on Linux should work.