This is really cool, and Leaky Paywall is a very cute name.

Ever since I launched Paid Memberships Pro, I've wanted to mimic the NYTimes 5 free articles per month system. We had a PMPro client pay us to build this for them recently and a very simple version that will work with Paid Memberships Pro can be found here:
https://github.com/strangerstudios/pmpro-limit-post-views

For users that aren't already using PMPro, can use Stripe, and just need this specific feature, this plugin looks like a gem.