I see where your going with that one. It looks similar to my relay idea but instead of switching the common wire for the bulbs over to the pin you are using diodes so it doesnt have to switch anything to run a test, all it has to do is turn 1 bulb on at a time. It looks a little confusing at first but its starting to make sense now. All the bulbs have to be turned on and you only turn off the one that you want to test. If i did use this method then i would have to be carefull how i programmed it. If all the boards turned all the lights on together i think it would be enough to blow the transformer.

To be honest i think i prefer Acetronics' idea. Its very simple and requires less components/board space. As long as a single bulb can draw enough current to turn the switch on then it should work