I created a copy of the fieldgrass01.nif, opened it in Nifskope, and changed the texture path to a custom texture. However, in game, it reverts to the original fieldgrassobj.dds texture. Anyone run into something like this before?

I have not seen this problem myself when retexturing, but I hade some other problems to get the proper textures to show. The problem was related to the file path for the texture set in nifskope. The original texture file path is texture/landscape/grass etc and when I click in nifeskope I get a full file path starting from c: and that will not work for me. Other have the opposit problem, they need the full texture file path for it to work. I don't know if this is related to your problem, but it could be worth looking at.

Unfortunately, after troubleshooting and trying many different things, it appears the problem is deeper than this. This problem, so far, seems to only affect grasses:

You can open a grass model in nifskope and change the path to anything, but it will not change the texture itself. It seems that the Skyrim engine chooses a grass's texture based on the name of the model (i.e. the name of my model was ferngrass01, so Skyrim used the default texture it had tied to ferngrass01). If you attempt to make a new grass .nif, it will not load in the Creation Kit, or Skyrim, unless it has the same name as a default Skyrim grass .nif (i.e. fieldgrass01, ferngrass01).

As of this posting, it is impossible to use custom textures for grasses. You can change the model, the size, the density, and other variables, but the engine will not allow the grass texture to be changed.