My group's Techpriest underwent 'Cybernetic Resurrection' (from Inquisitor's Handbook ) a while back. Last session he got shot in the leg pretty badly, and our party's Psyker healed him. I didn't think about this at the time, but later it occurred- Cyber-Res is basically the full-body 'Robocop' treatment, right? So, all of his limbs are essentially cybernetic replacements, which shouldn't be 'healable' with Medicae or Psychic Powers, right?

Well it says "You may no longer be healed through the use of the Medicae skill." so perhaps the psychic power was an oversight in the rules.

I can only presume that you should heal the same way you would with the Medicae skill but using the character's Tech-Use instead of Medicae.

Either with a Cyber-Rebuild or Full Ressurection, eradication of all flesh isn't happening. In Cyber-Rebuild, lost limbs are replaced. In the case of Full Ressureciton, organs and vat-grown body parts are harvested to reconstruct the 'core' of the person, with embedded implants. No defined implants unless you as a GM said so - which means, unless the leg was specifically a cybernetic leg, the psychic power would have worked just fine.

Perhaps make it similar to the augmentists form rogue Trader, and only the tech-use skill can 'healed' them as they are mostly metal at this point

Well, since it's a psychic power, I'd handwave it as simply rebuilding destroyed circuits and mending steel as it would do with living tissue, if you think about it, it would be far more consuming to give energy to cells to replicate controlling them in order to not build up a cancer than to simply mend broken cables, though of course, I'd not allow a character to "heal" broken circuit boards, the intent of healing a teammate, and rebuilding a board are far different. (at least to not allow cheese)