In any case, we're talking about a small fraction of wOBA's calculation and honestly it's not worth debating it this much.
Ground rule doubles are doubles. Infield singles are singles. Adding a separate category for them doesn't make much sense. Again, it's all about run correlation. If you can show that separating those two types of hits out and putting them into the formula for wOBA makes the statistic correlate to runs better than it currently does, then I'd be all for it.
The thing is, it probably doesn't.
Another problem with it is record keeping. I'm not sure they actually keep track of infield singles and ground rule doubles. The data is probably there, but it might be more of a pain in the *** than it's worth to put it in useful form. Retrosheet might be able to do it, but I don't know if that's even being used anymore.