Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
Yes, it possible to restrict access for the single blog entry page in Payplans 4.1.
Perhaps you can give it a try our Payplans RC version and see how it goes.
https://stackideas.com/forums/payplans-4-1-0-rc-1
But I would encourage you install into your development site to test this out.
There got 2 type of restriction for this :
1. You have to create "Content ACL" app
2. Or if you want to restrict certain blog content, then you have to properly need to add those syntax code into the content.
{payplans plan_id=1,2} blog content {/payplans}