I was also looking into making boards to do just that, but the problem I ran into was the sheer variety of types that it would take to cover the market. Also, different systems do the lighting different ways.
The controlled lamps are a different story. As an example, on my Black Knight it runs 6.3v AC power for the GI circuit. I can just throw in a bridge rectifier, an optional smoothing capacitor, and call it a day.
But the controlled lamps are a different story. System 3-7 Williams boards use 18v DC power for the controlled lamps, and run it on a pulsewidth modulated circuit to keep the effective voltage down to 6.3v. Not sure yet how to deal with this in an easy to apply add-on circuit.
But in the past couple months new design LED's have come out which are supposedly compatible with this. I still think it's a kludgy fix to make complicated LED's in order to deal with this, but if it works.... it works. The Noflix Plus bulbs even have a capacitor to really smooth them out, though they are spendy spendy at more than double of anybody else. The Ablaze bulbs that pinball life sells, I don't know if they have the capacitors, and they also don't offer the warm white which is really a necessary color for older machines. As to the Cointaker LED's, I need to contact them to ask about that sort of stuff. I have other issues to deal with before making a choice, and it's a rapidly changing market.
The other option, instead of LED's, is to go with a lower powered incandescent bulb. The common recommendation is to use #47's instead of #44's, as they draw 40% less power. Colored lamps I haven't seen in #47's, but are easy enough to make if you really want them by using the various clear transparent paints on the market.
-Hans