For those who don't know, an auxlang is a language that has been constructed with the purpose of being used by people who don't share a common language. They are often designed to be neutral and easily learnable (for example, I'm sure most people have heard of Esperanto; this is a pretty well-known auxlang). So, auxlangs are meant to be a neutral solution to finding a common language to use with others. As much of a constructed language enthusiast as I am, I'm still not much of a proponent of auxlangs, and here's why: as a Language Planning student, I want to encourage people to learn and speak endangered and minority languages. So, when my passion in life is the preservation of lesser-spoken languages, why would or should I support a language that has been designed with the intention of being spoken internationally? It's somewhat of a conflict of interest.*
Personal matters aside, would an endangered language auxlang actually work? I'd have to say that I don't think so. To explain my point, I'll give you a little example: my hypothetical auxlang, Brėthoneg.
Brėthoneg is designed to be a hypothetical auxlang for the Brythonic languages, i.e. Welsh, Cornish and Breton, the latter two being listed as endangered by UNESCO. This project started as an idea to make a language that would be intelligible or easily read by speakers of any of the Brythonic languages. When I realised that this was pretty much impossible, it became a language that should simply be easily learnable for those speakers, with plenty of room for regional variations (one example being that <r> may be pronounced according to the speaker's native language, i.e. /r, ɹ, ɾ, ʁ/).
Simple phrases are easily intelligible in the standard forms of the languages:
English: I have a book
Welsh: Mae llyfr gennyf
Cornish: Yma lyver genev
Breton: Ul levr zo ganin
Brėthoneg: Ma lyvr geniv
English: Hello! My name is Rhian. What's your name?
Welsh: S'mae! Fy enw i 'di Rhian. Beth yw dy enw di?
Cornish: Dydh da! Ow hanow yw Rhian. Pyth yw dha hanow?
Breton: Salud! Rhian eo ma anv. Petra eo da anv?
Brėthoneg: Deidh da! Ma anu ėw Rhian. Pėth ėw da anu?
...and so on and so on. If you look closely, the biggest problem with Brėthoneg is that it is ridiculously similar to Cornish. So if I've designed an auxlang based on three languages, and it turns out to be structurally almost identical to one of them, why not just learn that one, and not waste time on a conlang? I can only imagine that in a larger group of endangered languages, a similar thing might happen, or all the words would be so different that there would be no way to find a neutral solution.
Coming from a language ecology standpoint, I think it's more important to encourage the learning of the endangered languages themselves rather than to suggest a new one for people to learn. I don't think that anyone would want to learn an inter-Celtic language, simply because Celtic-language speakers already have a common language, as is the case with most speakers of endangered languages. Besides, I think it takes a certain kind of person to take an interest in an endangered language, another kind to want to learn an auxlang, and I think that it would be a very rare breed that would want to learn an auxlang for endangered languages.
So, would they work? As with any auxlang, it's a nice idea, but I really don't think they would catch on. A nice conlang project perhaps, but I wouldn't expect too much.
* (I am aware that auxlangs are meant to be used as a second language, but many endangered languages' only hope is to spoken as a second language, too.)