Since we conveniently have an inside man, which one of the payroll providers would you recommend us use when we start a startup?

Use ZenPayroll. They've had a good relationship with Zenefits since the start. And they're both YC, so it's unlikely that will ever end.

I'm sure they can't publicly pick a partner over others. Someone at Zenefits should use a throwaway account :)

Why not buy up some other domains and setup forwarding to your Zenefits employees. This way ADP can't block you and your customers still get the benefit!

That would be pretty easy to figure out. You can't really keep that one a secret because you have to tell your users to switch it in their accounts. A simple dns lookup will tell you if it's pointing to zenefits.com. And if ADP already knows the accounts that use zenefits, I'm sure they're just going to watch them to find patterns. Even if you bought 10 domains, your customers still have to share them, and ADP still knows who they are. You could probably do that for brand new customers, if you keep the number of people on the same domain to a minimum. Seems like a lot of work to me.

Why not make the vendor an email address on the customers domain? We do that all the time for contractors.