Good thoughts, here's some structure.
1 - A simple addon to remind folks to cast DI, and to give it to the toon in group who would theoretically give the best rDPS increase. This would be "casual" mode, and would let the 80% who really aren't progressing improve overall play a bit. I'd personally use this in 5-mans, instead of having a TellMeWhen icon configured to remind me when I don't have the buff, and a scrap of paper with the theoretical "best" classes.
2 - A complex addon that integrates with Skada/Recount and tracks the delta in pDPS and rDPS that can be attributed to the allocation of DI. This would be a simple looking addon, but actually might be rather hard to do. I wouldn't use this, since I'm not progressing, and my big concern is "do I have DI on somebody who will benefit me (and the group) at least a bit?"