Y`know, any kind of security protection can be leaked - passwords, coupon codes, url`s to hidden pages, etc. I think the point isn`t to make access so difficult that it could possibly deter sales, rather to make the customer feel special about getting their discount. I say make a public page that is not linked to but keep it hidden from the crawlers and don`t publicize it (beyond this one company).
I think that you just need to put trust in your customers. If a few leaks get out, then so be it. Simply keep an eye on things. Do a search every once in a while to see if your "secret" is out. If so and it`s hurting regular sales, make a change. But also be prepared for that, like changing the url or coupon code that this company can also revise on their site.
As an entrepreneur, it`s easy to believe that all your customers are going to be scrounging the internet for the best deal possible. I don`t think this is the case. Some people will do so but it isn`t the norm. And for outsiders that might that happen upon the discount, let them have it. Look at it this way, you`ve decided to reduce your price to a select group based on the potential of some quantity sales, right? So if a few more than expected get in there, it`s still part of the quantity. You might not have made that sale otherwise.
I believe it is a mistake when rare instances (like discount leaks) drive business decisions that should cater to the majority of your customer base. Most people will still end up at your regular site with your regular prices. Trust in that.