ASTA said
I don't see why not. Roleplayers already do this with a rather infamous example of alien life: Humanity.
Thing is that everyone's a human on this forum.
EDIT: However, player interest might be difficult to garner or retain. When it comes to aliens, everyone has their own take on them. Some players may find it annoying that their creativity is somewhat suppressed by the whole 'One Species Only' rule.
The problem I see with the one species policy in Empty Horizons was that in practice it was highly unstable since it turns out players don't want to compete for "special" spots. The policy was broken since it ultimately implied only one player can be alien, something of which would have been the case with the atompunk race you had for instance. However would creativity be stifled if instead of having any player who wants to make the species, the GM[s] designed the species instead?
The challenge I see would be making a alien race that is distinctly alien without making the alien race unappealing to utilize by other players. Especially since the players would have to take into account the biology of the alien race. For instance what sort of traditions a egg laying species may have likely would be different from one that gave live birth.