Do you mean as part of questions/answers/comments or as actual paid advertising?
I think you'll find that the general consensus will be that using answers/questions to explicitly promote anything (be it your product, your blog or your Area51 proposal) will be frowned upon and likely voted down/flagged as spam. Questions are for getting answers and answers are for providing the best/most relevant information possible. I can't see how a link to an Area51 proposal can possibly fit in either of those categories.
If you meant through paid advertising, then I can't see any problem with it at all (although I doubt that's what you meant).
I feel for your issue, but I don't think spamming SO is the answer.
Edit: One possible idea would be to allow free advertising (in the same manner as the open source advertising that already exists) to cycle through nominated Area51 proposals to try & help to drum up interest... Seems like a good use of advertising space for SO (inc) as it'd be helping to cross-promote and help to grow the SE network.