Ask HN: Is there a Yelp for Special Dietary needs? Can we build it?

I hope you forgive me if this all sounds rather naive, as I'm new to both app development and diabetes. I'm curious about either finding or building a tool that would be like Yelp, except for special dietary needs.

I personally do not have diabetes, but my girlfriend just got diagnosed with pre-diabetes and gout both in the past 2 months. The hardest thing for her is getting used to what do eat and what not to eat given the new diet. I am hoping for something to merely help find (via crowdsourcing) dishes at restaurants that are specific to different dietary needs. Think "Yelp", but for particular dishes at restaurants that friendly for diabetes, gout, vegetarians, kosher, halal... Users that categorize their own dietary needs will post pictures/descriptions of the foods they eat, allowing similar users to discover them too.

Is there an app like that currently, and if not, what would be the best way to create one for Diabetes patients at least? Honestly, could such an app gain the critical mass it needs to take off, at least in a dense urban environment like San Francisco? Would people be interested in actually using such an app? Would it even be useful for diabetics? If so, do you think it would take a lot more resources than a single dev working nights and weekends? Marketing costs? I'm not too concerned if other apps like Yelp or Urbanspoon take this segment over, as that would have also achieved my purpose.