Just some thoughts: Anyone can weild Unborn weapons (as of second printing), so I'm guessing you mean take the Unborn option of having it built into them? Sounds like a good start.
My original specialty thoughts was they could replace their own with it later as a new "cheat death" option, but putting rules to that seems kind of difficult (prereqs no longer met, max skill ranks change etc).
Building on your idea, if you don't mind:
The flesh is weak, and fails. The machine endures and is strong.
Benefits: You replace a portion of your body with magically or technologically animated machinery. You count as always wearing Light Fittings and may install a single weapon as if you were an Unborn. Further, the lower of your Strength of Constitution increases by 1.
Special: In some campaigns this makes you a sort of pariah. The DM may also make you Revilled, but if he does you gain a Natural Attack related to your replacement components (ex: an Arm gains Slam or Claw, a Leg gains Kick or Trample).
Through magic or technology, you have thrown off the shackles of evolution to become the creature you choose to be.
Benefits: You count as an Unborn (or campaign equilivent Robot) for the purposes of any prerequisites (including specialised gear options). You also gain Burden of Ages and must now be healed using suitable Craft checks instead of Medicine Checks (as well as Tinker instead of Cure spells, if applicable). Additionally, you count as always wearing Heavy Fittings (these replace your previous Light Fittings).
You may choose to embrace your new nature once, when you level, gaining the Cyborg type. If you do, your starting Action Dice decrease by 1 and you find it hard to care about pathetic meatbags, suffering a -5 to all Medicine Checks.
I'm inclined to make them Gear feats, rather then Species feats. But that might just be me. Also, typing that on a cell phone sucked, so I'll type up the cyborg type later if anyone wants. In some campaigns Construct will be fitting, but I don't like it as a type (it's way butch). The character should be affected by Stress damage for a start, since it doesn't have a new mind or anything, just a new case. Also other tone downs to it's overall package. YMMV and all that though - the feat shouldn't explode because you let it have construct, though I'd consider adding something to it - like -2 Charisma if you do change it to construct. Tried to keep it setting neutral so it could be steampunk or chrome.
EDIT: Crud. Burden of Ages doesn't slow natural healing, which was what I wanted. /facepalm.
Is there a quality that halves natural healing? Sort of Fast Healer but backwarwards?