I'm not sure if this is intentional, but you cannot grind any of the other vanilla meats, and I'm guessing no mod added ones either. The way I would do it is to check the following in the order listed:

1. Check if instanceof ItemFood
2. Check if isWolfsFavouriteTreat (or whatever it's called, usually this is used as a "is meat" field)
3. Check if it has a smelting recipe

If it becomes a problem for a few meats you could add a configurable grinder blacklist.