Depends on what you mean by Cryptographers. There are various kinds of experts in cryptography: academics (mathematicians, CS theorists, computer science and engineering academics researching in more applied areas of crypto), and security and cryptography experts from industry.
If you mean academics like say Oded Goldreich or Shafi Goldwasser (people who write academic crypto papers) it is going to be very difficult. Based on my very limited amount of crypto knowledge I can say so far this site has not generated enough questions of the type that academics study to make them interested in visiting the site. You need academic researchers that would ask such question to attract the top experts, and even if you get such questions, it will still be very difficult to attract them (they don't like time sinks that would take from their research time). Advertisements will not make it, you need recommendations from people whom those academic experts know personally.
On the other hand, if you mean expert Cryptographers who are not academics but say from the industry or the hacker community, I think there is a good chance of attracting them by advertisement. There are many online security forums, mailing lists, blogs, ... and advertising in those places will attract experts that would answer the questions and people who would ask quality questions that those experts would like to answer. Another possible place is news sites like slashdot and hackernews. If you have a good quality question or answer share it there and that should help a lot with getting more experts and good questions.
Comparison with MO is not a good one IMHO, MO was started by people who were or personally knew some of the top mathematicians from the begging. That is not true about crypto. You should have had those academic researchers on board from the start, it is very difficult to get them later, right now there seems to be a handful of people who are crypto researchers here, let alone famous ones. It is going to be quite difficult to convince them to use a site started and populated by non-researchers. I think researches in applied cryptography are the group of academics that are most likely to be attracted to the site.
I don't think focusing on getting academics is a good strategy, there are many crypto experts in the industry and hacker community that can ask interesting crypto questions and answer them.
I am a moderator on cstheory, have argued previously on area 51 for two level model against one level model, also against this crypto proposal, and I still more or less hold those views, so what I wrote above is probably biased towards them.