I've seen questions about this before, but can't find them now.
There doesn't appear to be any way to allow restricted access to any products.

For example, I've figured out how I can enter Paypal subscription plans into Zencart (though they are still handled strictly through Paypal).
But this would allow me to track the sales through Zencart and to have them uploaded to ShipStation along with everything else.

I created a product called Subscription Sales with an attribute for each plan, and it's working well.

But it's visible to everyone.
I used bold type and big 'FOR OFFICE USE ONLY!' phrasing, but it's still not a really professional way to do this.
And inevitably someone is going to try to create a subscription with it no matter how many places tell them not to do it.

I was hoping v1.5.7 would have something to allow some kind of product visibility parameters, but I don't see anything. All I can do now is make it non-visible except when I need to use it and hope no one else tries to use it themselves during those brief periods.