I'm usually just a lurker here, but since I'm an attorney who specializes in insurance coverage, I thought I'd add my two cents.
The OP's idea would never work because geocaching.com is not an organization, it's just a website-based community. No lawyer would provide legal advice to such an open-ended group; you don't even have to be a premium member to read the forums. Often, a non-profit association which exists as a legal entity will ask members who are lawyers to work on legal projects; I have done that for a local hiking group. In those cases, the advice is provided to the association itself (and its officers), and not to the individual members. It is never the type of advice that can be published for the members' use. I can't imagine that any lawyer would agree to be "on retainer" to answer individual cachers' questions. A lawyer's own malpractice policy would prohibit him/her from giving advice to any individual who is not a real client (New York, for example, requires formal retention agreements naming the client).
As for the suggestion that a group of lawyers get together and assemble something that falls short of formal legal advice, this is also not likely to happen because of liability concerns. I believe that any lawyer who would agree to this suggestion, or any lawyer who would give advice anonymously, is not likely to give trustworthy advice.
With respect to the comment about a "geocacher's insurance policy," I'm not sure what this means. Who is going to pay the premium? Once again, no insurance company is going to issue a policy to an open-ended group. Geocaching.com cannot purchase a policy that will cover individual geocachers. A local non-profit group might, but there are bound to be many restrictions and it might not make economic sense.
If you really have a pressing legal issue related to proposed geocaching activities, your best bet is to join a local geocaching organization and ask them if they will sponsor the activity. At the very least, they are likely to have insurance in case everyone guesses wrong.