Adding the load resistor may make things more complicated than it seems. There are a number of games that use lamp boards instead of discreet sockets, and then you'd have to mix and match your methods for handling the anti-flicker properties.

That and it also limits the locations these LED's can be used. They wouldn't be useful in non-bally/stern machines, nor would they be useable in GI locations if so desired.

Not saying it couldn't / shouldn't be done, just saying that there are a few other things to consider to make sure other issues aren't created in the process. To be quite honest.... I often wish that LED's of this style were available on the market, and I've considered doing something along these lines for my own games as well.

I wouldn't worry about diffusion at all, as I've used very similar RGB LED boards in my PROC project, and they look just fine.

-Hans