In theory it's not that difficult to work out - you get a PC-link module and the Niko control software and you install something like 'Free Serial port monitor' and off you go. It should be easy enough to switch lights on and off and see what happens. I've used the same technique to write a network version of a software to send and receive telexes on an old Belgacom telex machine.
So even if I had looked at serial before I didn't spend the cash on a PC-link but glued LDRs in a U-rail, I then attached an L-profile over the U rail so so I could mount it on the module and I had the light sensors lined up over the status LEDs. I'm looking at 3 modules so I have 3 analog multiplex chips looking at 13 LEDs each so I needed only 3 analog in on the Arduino.
Dimmers are even easier to monitor - use a voltage divider to reduce the output to 40% so the 0-10v ends up between 0-4v so you don't kill the analog in. Be sure to have common ground between Arduino and the dimmer module.
The Arduino runs an AJAX webserver (see w3schools.com for info) that shows a list of all lights with a tickbox which indicates if they are on or off. The tickboxes are updated every 2 seconds. If you tick a tickbox that switches the light on or off which is done using simple IR. Dimmers have drop-downs 0-100% with 10% increments. (for IR see http://www.righto.com/2009/08/multi-protocol-infrared-remote-library.html - Niko is using RC5 and I can send you the code list if needed)
In the early days of PC-link I don't think it was meant to be used like this so I think you had to poll the module to see what status it was in and I think I read somewhere they didn't like that. This approach has no impact on the Nikobus at all even if I wouldn't mind having a 3D printer to make a nicer 'status-LED-sensor-attachment'.