You are absolutely right, Brother. There are not only precedents for Eldar bionics, but also a number of options for what they actually are. Here's 1:
Notice Fierme Direstorm in the bottom left corner has a bionic left leg (designed by Jes Goodwin himself, this ad helpfully tells us). It's painted metallic here, but taking a closer look, it looks a lot like the Wraithbone that Eldar Dreadnoughts are built of, with its smooth shape and those telltale studs on the foot. I vaguely remember that Eldar power fist on the model is also supposed to be a bionic arm...like its being a power fist was just a secondary benefit of it actually being a prosthetic, but I'm not sure. It's like the Eldar preferred finesse weapons like power swords, but they wanted to give Eldar the options for power fists like all other 1st ed factions (even Tyranids had bionic arms on Zoats, a unit that no longer exists), and to differentiate that I recall them saying that power fist was actually a bionic limb, to show they prefer not using them.
Now it CAN be metal, depending on the Eldar's situation. In 40k 1st ed, back when the game was called Rogue Trader, it initially treated Eldar as spacefaring traders (these would later be Craftworld Aeldari) or pirates and mercenaries (which would later be explained by Dark Eldar/Drukhari). Depending on their resources, they might trade for human resources, such as human bionics that they'll modify to fit their own bodies, perhaps. This would be seen as especially crude, I reckon.
(Disclaimer - the above is, as I said, 1st ed 40k...this was back when we had an Ultramarine Librarian that was half-Eldar, so it's not all canon. That said, Forgeworld and even recent GW stuff seems to draw a lot of influence from 1st ed.)
But as the Dark Eldar lore in particular was expanded on, it offered another option; Dark Eldar Haemonculi, flesh sculptors, could re-grow or graft limbs. Not sure if that was a technique/technology available to all Eldar pre-Fall, just mastered by the Dark Eldar afterwards, but replacing (perhaps improving) limbs is definitely something that comes up with the Aeldari! But it sounds like your Eldar army probably don't want to deal with these Haemonculi and the high prices they demand.
And, of course, Wraithbone, as you suggest. If their craftworld still exists but is almost empty, they probably have no other resources that they could use or even trade for except its very own Wraithbones. Thus, just as this Wraithbone keeps the wounded Eldar alive, these Eldar also carry the dead-ish Craftworld with them. It's a very, very neat idea (in fact, if I may counterpropose an idea, I'd suggest the craftworld itself was destroyed by the Necrons, but what living Wraithbone they found they grafted onto the surviving Eldar, not just for the sake of the wounded, but it's actually the Eldar bodies that keep those Wraithbone pieces alive, a truly symbiotic relationship).
In any case, outstanding question because even when you're thinking outside the box, you were looking for everything inside the box 1st. Good due diligence, good creative discipline, Brother.
Edited by N1SB, 06 May 2019 - 03:55 PM.