My end goal is to have a module on the right of the registration page that summarises the products that the client is registering for.
I don't think you'll be able to do this very easily as currently you can't create Internal Joomla menu links to CBSubs plan pages. You'd have to use External menu links, but module assignment does not work for External menu links with Joomlas SEF is enabled as it strips the Itemid, which is what allows module assignment.
Oh.. and I may be pushing it.. but is there anyway to split the registration into two columns?
No, not without using CSS and altering the display, but that could be tricky. Registration is dynamically created from available tabs, fields, and API calls so it's not so easy to just split it in two.