That also could be done by the same method:
In the category listing page (or any search pages), just check for pledge level of the user with earlier mentioned function. If its lower than what you would desire, then just abort the page with a message or any other way. Best would be to just avoid printing any result but render the page.
Note that you will still need to implement the earlier suggestion too, because even if the user may not see the product from category page, s/he may find it through other means (google etc) and just check out.