There isn't any way to do this automatically, though you could feasibly do it through a custom module or Workflow-ng mixed w/ a little PHP action. I don't know of any user based taxonomy access modules (maybe taxonomy access control?) but I know TAC Lite can do it role based... if that works for you, you might setup a role per category and sell the role promotions through the UC product feature.
Thanks for the reply. I looked at TAC earlier, but it doesn't really do what I want.
I don't have many taxonomies, and users don't need access to many nodes, so I've ended up creating a node per taxonomy and applying access to the relevant node for each product (I don't have many products either).
This will do as a workaround until I can learn how to develop this as a module.