I think that your assertion that the fluid in the can (wait one for further digression on
what fluid), provides a nice humidifying atmosphere for the bird's chuff is precisely the mechanism... I am reminded that one is encouraged to put a large flat dish of water into the oven when baking bread to make the bread crust just that little bit special... the oven probably is at 200 degC or so, and yet there is not a lot of active boiling of the water... but some of the water does evaporate and that's what does the trick.
So... if one really is providing only a reservoir of something that keeps the bird moist inside (contain yourself, BEagle), and it doesn't really boil, it seems to me that beer is better used to moisten the inside of the OiC the evolution, and water is perfectly adequate for the bird. Of course, if one had a reasonable supply of cheap brandy or the like, and used that, then the evaporation just might be sufficient to flavour the meat.