Can raw egg yolks gelatinized from being stored be saved by blending and for which uses?
Finally reporting back, once I noticed I hadn't written this up yet.  

The reconstituted egg yolks work, with a few quirks.

After blending them with water, I had let them sit for a while - to make sure all the yolk from inside the lumps had a chance to re-hydrate.  After a while (half hour or a bit more), I noticed the mixture had become thick.  I added a bit more water - I thought I had added less before, after all I didn't remember offhand how much volume four egg yolks was supposed to be and maybe I had underestimated them.  

All told, the yolks took up maybe half a cup of water more before I was really, really sure that it was more water than the yolks would have started with - and they were still settling at a thick paste, given time to sit.  At that point, I said never mind and just made the sauce.

The sauce in question used a cupful of cream and some spices.  I made it, heated the cream, added to the yolks and spices, then heated the whole until it until it thickened (as the recipe required).  It worked just fine, giving a nice, flavorful, runny sauce.

About a half-hour, forty-five minutes later, it had thickened again.

So, the egg yolks definitely did not lose their thickening power.  On the other hand, they thickened a lot more than regular egg yolks - four egg yolks in a cupful of cream (and perhaps half again as much water) would normally not set up as a thick paste... at least, it didn't the next time I made the recipe, it's usually fairly pourable as a sauce.

This was not a problem for me, in my recipe.  I didn't mind the thicker sauce (it clung better), and anyway it stayed thin long enough to serve and plate when made, and re-thinned easily with a few spoonfuls of water each time I tried if I had needed, or wanted, the sauce to be thinner - it just didn't stay thin - and it didn't interfere with the flavor profile at all.  Even with the extra volume, it didn't taste watery or weak-flavored (perhaps because the water was locked away in the paste).  And while it looked just a bit gritty or grainy, it didn't feel like it on the tongue.  So I was pleased by it, and the fact that the sauce slathered rather than poured just made it easier to control.

So, egg yolks that have dried out and gelatinized can, in fact, still be used - for flavor, for thickening if one doesn't mind a tendency to keep thickening up, for emulsification, and so on.