onCPayBeforeDrawSomething works with existing subscriptions. CBSubs Family Plans uses it to add its share subscription button for example. It has a $controlButtons variable which is the HTML of the already processes buttons and is the only way to modify them. This was explained in my below reply.
www.joomlapolis.com/forum/developer-members-support/244524-cbsubs-plan-css-classes?start=0#327337
There is no easy way to do what you're wanting. We already have a feature ticket to review per-plan unsubscribe support and will also look into adding some access permissions to it (e.g. moderator only unsubscribe). It is at the top of my TODO list after I'm done with CB Activity 6.0.0 (I cannot keep pushing its development due to other tasks I'm sorry).