products without categories

Here's the issue. I have product A, and I'll use an upsell mod to promote product B. However, I also carry product C, and if they aren't also buying A, I would prefer they buy C instead of B and have no knowledge that B even exists. Products B and C are very similar and would be listed in the same or similar category. What I want is for the only way a customer to find B is by the upsell from A, and otherwise they would find only C. Make sense?

Any ideas?

No, doesn't make sense to me. You want a product that's hidden until someone doesn't buy something? A link to the site and the products in question would help.

Since you obviously don't want B and C seen together, you can put them in different categories, and use the hideCategories mod from Free Addons to conceal the B cat from normal navigation.
Then an upsell/cross-sell mod active in A could link to B. It might take some custom coding to allow B or its link to show, or it might work straightforwardly - try it and see.

B would be visible when just looking at A, and if you want it to only be visible after A is added to cart, that would be another bunch of custom coding.