I was looking to be able to get seeds from some mods when I broke grass (either long grass or grass blocks with the hoe) and only one mod seemed to have a single seed registered with the seedList so I made a minor change to the current code locally that I thought I'd share.

First I subclassed GameObject with one called WeightedGameObject simply to add an additional weight parameter to provide varying random values for newly registered seeds.

public class WeightedGameObject extends GameObject {
  public int weight = 10;
}

In HOJsonData I added a new array:

    public List<WeightedGameObject> grassSeedsList;

which now lets me add to a custom json file an entry like:

"grassSeedsList": [
       { "name":"etfuturum:beetroot_seeds", "meta": 3, "weight":5}
 ]

Then inside the JsonModule's init() method in the loop processing the HOJsonData, I added an additional code block:

        if (h.grassSeedsList != null)
        {
            for (WeightedGameObject gameObj : h.grassSeedsList)
            {
              ItemStack seeds = gameObj.toItemStack();
              if(seeds != null ) {
                MinecraftForge.addGrassSeed(seeds, gameObj.weight);
              }
            }
        }

Now these seeds will be available when breaking long grass or right clicking the grass block.

Cool, you can submit this as a pull request if you'd like.