I've been wondering if it would be possible to have a sign-language that would work between two or more alien species with different hand-shapes (numbers of joints and digits and such). Would such a language even be possible, or would it suffer from its users lacking a shared physiology?
On a side note, it was lovely to find this blog. Your work is fantastic and never fails to liven up a setting
Chances are it would not, and a secondary medium would have to be chosen—and I have a great example. In the grand scheme of things, humans are aliens to non-humans, and I happen to know of a sign language created for radically different beings called Rikchiks. Humans can't sign the Rikchik language: It requires 7 tentacles. We can't even approximate it. What we can do, though, is (a) understand it by looking at it, and (b) communicate with Rikchiks by using the written form of the language. It's not nearly as fast, but it's possible. The same would be true for a Rikchik using a human sign language: possible for them to understand, but they wouldn't be able to sign it back effectiely (they don't have hands, and so can't do handshapes).
Hopefully that makes sense. Remember that you can always understand a language without necessarily being able to use it perfectly. Han and Chewbacca from Star Wars are a nice example of that.