On one site I work on, the html block (interfacing with a PayPal side PayPal button) was added as a separate block of its own and has no integration with the underlying store and is processed independent of the ZC system.

I've gotten to information related to this creation of a reoccurring payment button through multiple different searches all indicating that mostly still going to use a PayPal created button and associated html, though the information at the following link nearly suggests that it could be "generated" using an alternate path.

That said, most of the things occurring are simply posting data to payPal so it almost seems like the post information and associated form address could at least in part be modified "temporarily" to address the reoccurring purchase and then separately process the one-time purchase and maybe collect all of the information in the site.

Perhaps I'm just thinking too big, dunno.
https://www.paypal.com/pdn-recurring