i'm not sure there is any practical use for bypassing IP on such things as bluetooth, when most bluetooth devices of any recent vintage are capable of doing it. i mean, it's just a PTP connection like a USB serial bus at the level you usually would use it for this, but PAN is available so why complicate things by using a different flow control protocol (and that's a bit of a nightmare in itself, it's why QUIC still mostly follows what TCP does).

of course, being that it's a point to point connection you can just rely on the hardware based flow control for a duplex connection, or you can implement RTS/CTS in your protocol but ... just why not use TCP/IP when you can