Great idea such a mentoring system
It would probably make sense to establish a website to enable people to make contact, or even as an addendum to pprune (we're all here already anyway..). The pprune profile allows for some details of experience to be put in, shouldn't be too hard to get a little section of the site going that would allow people to offer/seek general advice.
I wouldn't worry too much about the legal implications of mentoring, a simple disclaimer signed by all or as part of the statutes of a club/website membership, that the PIC is ultimately responsible for his or her actions and all aspects of planning, would do nicely. Moral/ethical consideration are a different kettle of fish, but really within the realms of responsibility of the individual offering to provide advice. I would say quite simply if one has the experience, it will more than likely do more good than harm to share it with someone who'd otherwise have to do without it.
Aero Clubs are probably alive and working in more rural areas, when I look at Moorabbin for example, there's really no infrastructure for newly qualified pilots, unless you happen to be affiliated with the Royal Victorian Aero Club, and when I was a member to train there (which didn't last long because of a couple of abysmal instructors), the Club or its members and activities weren't that visible sadly. Seemed like a bunch of old men (no offense, getting on myself
) just using the facilities to go flying, without much of a community feel.. I may be wrong here as I didn't spend too much time there..