Employees of Partners HealthCare (i.e. Harvard Longwood, MGH/McLean) have all moonlighting insurance covered by the same insurer who insures us for in-house training. Assuming a resident is in good standing in his/her program, you literally fill out one piece of paperwork (a single side of a single page) on which you list all of the potential moonlighting locations outside Partners and have the Chief of the department sign the bottom, and you're all set.
I can't comment on the "tail" coverage and other specifics, but I would assume that it's a reasonable insurance package as our residents who moonlight do not obtain additional coverage.
From what I understand, it's one of the perks of these programs, though I imagine other programs have this perk, as well.