Well, its an archaic word in origin and its true meaning isn't oft-understood, so I would refrain from using it in an official capacity. However, boards aren't usually an official capacity in which case I would use such a word with its commonly-accepted meanings, even if those meanings aren't Merriam in nature.
Around here, 'penultimate' means second-to-best, but loosely. We even have stores that use such a name, such as 'Penultimate Costumes' which is a local store that specializes in clothes and costumes, but in a thrift-store sort of way.
However, it could easily be a more local definition. It's proper for where I hail from and I figured other people would understand it to mean the same thing. Even if the definition wasn't "official" in the way of doctionary.com etc.
But hey, you got the gist of what I was saying and we've certainly spent more time on a single word than we needed to. =)