Just wanted to follow up on this. I'm pleasantly surprised how well this works, considering my environment. I bought a WS12A switch module and an ir543 controller and also a ur47a remote control (ebay item which hasn't arrived yet). The ur47a is designed with special buttons on it to control a number of X10 devices. I am going to have my URC RF20 remote learn from the ur47a because my experience with pre-programmed codes has been spotty. In the mean time I did put in a pre-programmed X10 code that I found in the RF20 manual. This gets the dimming function to work (vol+ and vol-) but sure enough the channel buttons, which are supposed to turn the lights on and off, don't do anything. Neither do the number buttons, which I'd need to type the module number. There is another X10 code in the manual that I haven't tried yet - maybe it will work better.
The first thing I noticed, and this has been reported by others, is that the ir543 led light gets stuck on when it's in a direct line with the TV screen. When I moved it out of a direct line (but still in the same room), it goes off but the remote control range is severely limited to about 2 ft. With the TV off, the remote works fine even 20 ft away.
My plan, which I didn't have high hopes for, was to put the ir543 in an A/V closet, the door of which faces the TV. The door is louvered, but luckily it blocks the TV noise that bothers the ir543. The ir543 works in the closet because the RF20 sends RF to an RF/IR converter in the closet and a blaster retransmits it. The circuit in the closet has an outlet with two power strips, into which is plugged a computer, cable box, avr, dvd player, cable modem, router, switch, access point, IR blaster, and maybe some other junk I can't remember, along with the ir543. This circuit is on a different one than the lights I am trying to dim and, as far as I understand these things, the circuits are on different phases as well.
Despite having no filters, repeaters, or couplers, the thing works really well (at least so far, knock wood).
Some other observations:
Prying off the toggle plate was a bit nerve racking. It didn't pop off easily and I was afraid I would end up cracking the plastic somewhere. After all that I ended up leaving it at A1.
There is some buzzing at the switch and it's loudest at about 50% of dim. Although it's noisier than the one I replaced, it's not as loud as I feared. I'm not crazy about the mushy feel and having to re-press the switch plate to reverse the dimming direction, but I suspect I'll get used to it. I haven't got a proper switch plate cover yet but the heat coming from it does not seem any worse than the dimmer it replaced.