I don't think there's a service like that, but let's attempt to talk through who could potentially use it to figure out if there's a market.
1) Would a developer who is part of a workforce in a company where there are other developers use this? Chances are, probably not - he has coworkers whom he could ask to help him work through a problem. This is more efficient than hiring outside help - his coworkers probably understand the codebase and the problem domain much better than he does.
2) Would a student developer in a school use this for his schoolwork? Chances are, probably not. Schools usually have tutors, and the instructor is usually available. Also, students are usually low on money to pay for services like this.
3) It sounds like you are what I call a "lone-wolf developer". You consult for projects as your main source of income for companies that don't have on-staff technical talent (otherwise you'd ask them), or are a single founder working on your own project (or, if you have cofounders they are completely non-technical). You would definitely use this service. How many people are there like this?
The reason why your post caught my eye is because I often find myself in the "mentor" role that you are describing, and have often wondered that question myself.