compromises, compromises..... every time you turn around.
Since I'm getting power out of the wall in any case, I decided to just simplify my life and control the ac before the 12vdc transformer. But the PWM wall plate dimmer I had on hand doesn't get along with the switch mode power supply transformer - I guess that makes some sort of sense even to a circuit dimwit such as myself.
I got out a 12vdc 1 amp regular old stacked plate transformer and things work just splendidly. I was so hoping to use the 12vdc switch mode though since this project is supposed to be portable and I just added about two pounds of weight.
Oh well.