@evan It is definitely possible, and implemented in my project Mitra. Here, "subscribers" and "followers" are different categories, and subscribers-only posts are published in a private group that uses mentions for targeting and therefore compatible with other popular micro-blogging platforms.
A variant of FEP-0837 protocol is used for federated payments. People on servers that don't support FEP-0837 can make a payment by navigating to a special payment page.