Gotcha.
I must point out that a generic multiparty relay isn’t necessarily a limitation of a protocol (SPN in this case) but of the implementation (that is, Portmaster, the client, which runs SPN could implement multiparty relay).
In case of MASQUE (Apple’s Private Relay over Akamai & Cloudflare), the protocol itself has inherent guarantees (like Tor’s Onion routing / I2P’s Garlic routing), so may be you’re looking for a multiparty relay of a specific kind?
I am curious because there’s even more ways to “multiparty relay”. In my toy app, for example, DNS can be relayed already (pretty easily over more than one party). Relaying TCP & UDP that I’m working on (WireGuard relayed over another WireGuard; specifically, Proton over Amnezia) has turned out to be an enormous pain (on top being fragile), and so I was mulling whether I should spend more time pushing that over the line…