I haven't been keeping track of it, but is there still a "looking for/selling freelance work" thread that accompanies the "who's hiring" thread at the start of each month?
If so, that's probably the most appropriate place for it. (If not, you could start such a thread at the same time.)
There have been several front-page threads of people offering free office hours and/or some kind of free service, but even that got some argument over whether it might be too self-promoting of the posters.
Generally, there are lots of companies and individuals that want to sell services to HN users, but we really aren't interested in seeing them as posts unless it's something really ground-breaking in its own right.
[Edit: side note -- if you have lots of good advice to give to startups, you might offer it in discussions here -- this can give you a sort of portfolio to share with prospective clients. And in at least one case I know of, a frequent HN commenter was basically begged by many clients to start doing consulting like you're talking about, because he so clearly had advice worth sharing. YMMV.]
