Yes that's doable, but not with Basic sorting. You will have to use Advanced sorting and supply your own SQL. The Advanced sorting just directly adds to the database queries ORDER BY clause so keep that in mind when formatting your SQL.

I thought I'd ping the CB Team first: Could you offer some guidance on how I'd configure this list? (maybe some tips on what table column names to use?) Thanks much!

We do not provide coding assistance outside of Business memberships, which are significantly higher priced to cover what it actually costs to have someone write custom code for you. It's just constantly using an extreme amount of our time for basically free (I hope everyone understands we have not changed our prices in 5+ years despite the, saying this mildly, rough economy). Normally we only provide configuration help. So for example configuring an auto action or telling you want parameters to adjust. However raw PHP and raw SQL do not fall within that. I hope you don't feel like I'm attacking you or being hostile with this reply as I'm genuinely just trying to give a reason as to why we stopped providing custom coding help and if you do feel that way I deeply apologize as that's not my intent.

So with that said the best I can offer you is older examples we've already provided and also explanations of database structures. Below are examples of how to sort a userlist by a plan.

www.joomlapolis.com/forum/developer-members-support/244413-sort-list
www.joomlapolis.com/forum/255-developer-members-support/235683-solved-cb-list-and-advanced-sorting-with-cbsubs

Below is the table relevant to subscriptions.

_cbsubs_subscriptions

The `status` column holds their current subscription status. For active subscriptions this will be "A".
The `user_id` column holds their Joomla user id linking the subscription to a user.
The `plan_id` column holds the plan id (as seen in CBSubs > Plans) for which they are subscribed to.