Hey everyone --
I have a client who wanted this feature and so I've put together a hack that has this net effect.
Basically, my hack excludes individual products from group discounts. It does so by adding a read-only attribute to the item.
Because you can globally apply a given attribute to all items in a category via the attributes manager, this has the same effect as excluding all items in a category from group pricing. As an added bonus, each product's catalog page will display the fact that the item is not eligible for group discounts, which prevents your customers from complaining when they see the discount they are getting is not for the entire cart.
Because I have just finished this hack and I need to be awake and on the road in six hours, I am not going to describe the hack now. However, I plan to discuss this hack in depth in a follow-up post, which I will probably put together on Saturday.
It's not a really complicated hack, but it is multiple steps and it does involve messing with an important core ZenCart PHP file. I don't want to give it up to the public without describing it in full, so you'll have to hang on a couple of days.
Also, again, this is a hack, not a mod. The difference is that you can't simply plug this item in, you need to do it manually; and if you update ZenCart, there's a good chance you'll wind up replacing the hack.
All that said, a final bit of good news: I'll be releasing this hack under the Creative Commons Attribution / Share-Alike license, which means you'll be getting it for free. (It also means you'll be using it at your own risk.)
Look for my post over the weekend.