You could try a "widget" on the lawyer's site which facilitates getting generic questions answered for free.

The idea being that in each practice area, there might be a handful of questions that they get asked frequently, and would commit to answering one-time. It could be used to qualify the web visitor (always a good thing) while satisfying the visitor by providing them an answer. Of course, the challenge here is that most lawyers might only be comfortable providing such watered-down generic advice, that the answers themselves wouldn't be very useful. But this way, you could provide value to lawyers somewhat comfortable with online discourse, while building up content. With enough lawyers and content, you could then expand the service to build towards your larger vision.

But as John has mentioned, many entrepreneurs have and are actively trying to win with this type of idea and have often struggled. CaseText is a recent YC grad that is doing some interesting work in this area.

Happy to talk through your product implementation.