Good Morning,
I contacted my LQS sewing machine repairman, whom in turn contacted the school where they instruct in sewing machines. The following is the instructors response...
"You are right! She can add the dimmer to any non-electronic machine and it will work just fine. I would recommend using plugs to the foot control cord only that way the light is not affected. As it says in the post, on older machines with a carbon foot control, the foot control is the equivalent of a dimmer.
You may want to check her Janome foot control to be sure it is working well and offers a full range of speed from starting to full (quite often the carbon will wear and the machine will jump to full speed from half speed)
Hope this helps,"