SUMMARY
I would like to be able to subscribe to DBus signals from a KWin script, which seems to not be something that is possible (last time I looked at least). Alternatively, it could equally be accomplished by being able to create DBus services from a script, either way would work.

As far as I can tell the current state is that it is hard to send any sort of programmatic message to an installed KWin script.

The essential use case here is being able to invoke KWin actions from another application. For example, a use for this would be to raise a window by PID using a command line tool: you create a script which stays resident in KWin and then you invoke a DBus service to send a message to the script when you want to raise the window.