I don't think that would work. We don't know how far his soul can move from his body, or how fast it can move. It's also probably not very subtle (flying spectral ghosts typically aren't) and Brook probably doesn't have the kind of long-range vision a naval scout would need (because he doesn't have any eyes YOHOHOHOHO). Mone on the other hand does appear to be mobile and observant (she got a better look at the SH than they did at her after all), and would probably be a great scout for the SH.
Not that I support her joining at all (she's had 3 scenes total, 2 with lines and absolutely no reason to join), but she'd fit the positoin well if Oda does decide to put her on the crew.
We do know after he died his Soul wandered around for an extremely long time in the foggy sea searching for his ship. If anything he can go as far as he possibly wants to. Mone looks cool, but even Sanji could fly off and scout things if he wanted to.
And Luffy does not count himself in the count.
jinbei's case is like another vivi. so we only have 9 so far. the next member will have some cape/hood or something similar.
Jinbe is the same only in the sense that they are "leaving him behind" for a period of time. The difference is he can't stay on his home island even if he wanted to, he say's as much during one of the final chapters featuring him. Vivi stay's home and is an honorary member, people assume the same for Jinbe after Luffy claims the island.
I think some people fail to see that even with an Emperor's flag stapled every where, Jinbe still couldn't stay on the island. If Jinbe can't set foot on the island while being a rep of Big Mom, he certainly won't be able to do it as a rep of the Straw Hats.