I like the idea. So much so that me and a buddy considered this years ago.
two key hurdles.
One is updating. Specials change at bars faster than their own website can keep up. How will you gather that information? It seems there needs to be crowd sourcing. This let's the owners, or patrons update on specials. Maybe a thumbs up / thumbs down thing?
two is competition. At least locally we have Metro Times, Real Detroit, Yelp, foursquare, etc that all give local specials. Its a tough market
But, If you can get people to the site, and keep them using the site you are golden.
Keeping them using the site is easy, if the data and interface is good.
Getting them to it is a bit more difficult!
Ideas to advertise:
Talk to restaurant bar owners,
Put promotional gear in bars (stickers, magnets, etc)
Social media, of course, Facebook, Google+, Twitter, etc
Hold some early minor events, basically create a culture around your site
Go to local events, street fairs, openings, etc
It's easy to get a database on the internets... Getting people to use it and keep using it is tougher!
Good luck!
Let me know if you want to expand to Metro Detroit area! :-)