Jim,
Keeping track of which is which isn't easy, but SRXL is not S.Bus. The 4648 does support the Spektum Sat protocol, so if that's your hardware, that's the receiver input I'd pick. It's clearly a better performer than PPM, and the margin between sat and sbus is thin enough it's arguable. Not sure if the 4648 will even do PPM . . . it's always possible, but as you say, the documentation seems to imply no.
It's an interesting question though . . . How do you know what you don't know that you don't know?
The ever present Unknown unknowns. Many of the "undocumented features" are found by accident (wait, that works? it shouldn't.) or by exploration (it would be nice if it . . . well, lets see if it does!). Reports come in, people share, and we all benefit. To answer your question . . . experiment and keep an ear open for what others find out.
