Peter, I'll assume that all five prongs of the regulations' definition are met, but there is no compensation, direct or indirect. The "rubber hits the road" when something or other "hits the fan." If the plan has a perceived investment loss that is perceived to be attributable to the advice provided by the person who is donating her effort, would a lawsuit by the plan in federal court under ERISA survive a motion for summary judgement? Tough question, but my guess is that the uncompensated adviser would skate for the reason you have explained, although her legal expenses could be significant, and if she has an agreement with the plan that says she works for free, but is indemnified for anything, let alone legal fees, I think that is probably "consideration". But now the plan is in a position where, had it paid for the service instead of accepting it gratis, it would have had recourse against the adviser with respect to the loss sustained by the plan. I agree with QDROphile's suggestion that at this point the plan's named fiduciaries would need to think about their own liability for having entered into a defective service provider agreement.
If this is informal and infrequent advice, I think I would try to get more comfortable with the "primary basis" and "regular basis" prongs of the definition before hanging my hat entirely on the absence of consideration.