A Poison-type Umbreon can work defensively, but Dark was needed, since Psychic was horribly unbalanced and the other Dark-types, aside from Tyranitar (who was not in the beta) tend to be fast and frail.
That would've been really cool, even if I do love Umby as a dark type. I think it makes sense that they retyped him in the end though, to balance out the psychic type. Already mentioned that they were pretty broken in gen I.
I honestly want to know more about the Spaceworld Beta versions of every Gen 2 'mon. It's like discovering an alternate universe Johto, and the multiverse is already pretty well-defined as a thing in the Pokémon franchise.
This is why I don't think it would be the best idea to make an Eeveelution for each type. A "Toxeon" would seem redundant given Umbreon's poison references.
Instead, I think it may be better to create new evolutions (or temporary forms) for the Eeveelutions, adding to them the unused types. Umbreon's evolution/form could be Dark/Poison, Vaporeon's Water/Dragon, Leafeon's Grass/Ground, etc.