Retrofit Wall Dimmers

In the USA, there is the UPB (Universal Powerline Bus) which is a two-way system that communicates over AC wiring. I have used it quite a bit and it works quite well. I'm not sure if it is available for use with other electrical systems, but maybe you could check with the manufacturer.

www.pulseworx.com

There' always Lutron's Homeworks RF line or the Lutron Radio RA 2 which is now shipping and it's much nicer to use then the original RA. PC programming negates the need to run around, at least after they are initially learned into the system or if you're level 2 cert you can just enter serial numbers.

I'll second the motion on RadioRa. It's a very solid system, no special wiring needed. You could also, depending on budget, go for a full blown Homeworks Illumination system with an RF processor. If you have the ability to still run control wires, I would recommend the Homeworks in any case. But much depends on budget, Lutron is not cheap.

Rako

We've got pretty wide experience of Rako and I'd say it would do what you want but there is always a possibility of being bitten by some hardware unreliability.

We've got a showroom with over 100 circuits of lighting controlled by Rako and integrated into our AMX system. We've used the RAV232+ bi-directional interface for feedback purposes. The protocol is strightforward and properly programmed you can make it do pretty well what a decent Homeworks system can do. It's VERY flexible and the team who make and sell it are fantastic. Our showroom works like a dream.

On the demerit side, they have had production batch issues and if you've been caught with a dodgy batch then you get a slightly less positive view of the product. For us, the one consistent annoying niggle is the number of keypads we get service calls about. We usually end up having to swap keypads that suddenly can't do scene 1 or 4 or won't turn lights off. Oh yes, and we've got a client (my in-laws...) who have lights that turn themselves on in the middle of the night.

If the architecture and budget can take it then I'd personally go with Homeworks

Will the PLCs you should put filters to block signal from coming in or going out on the main electrical service. If not you can have cross control from other houses using these thing on the same transformer if on the same letter code.

A long time ago one of my guys knew his neigbor had the older X-10 stuff installed in parts of their house so at night he would sit on his own porch with a plug in master keypad and start controlling the neigbors lights just to amuse himself. They'd turn off the bedroom lights to go to sleep and 5 minutes later he would turn them back on. They'd get up and turn them off and 5 minutes later he'd turn them on again, and so on........

The UPB stuff is pretty good though but I haven't played with it much. I go the RA2 if under 100 devices with no chance of expanding beyond that.