I am not against this idea. Infact, such a thing might be actually usefull late game when you've got so much mass and full control of a planet so that you can't use it all at once. Sure, you've got 2000 metal in storage that, with enough build power, you could consume in seconds. Once that's depleted, you have an extra, exhaustible supply of metal to use. I don't think it should be able to move though, there should be a "max size" of this unit, and it should be target-able by your enemies.
The biggest problem is direct fire units destroy wreckages currently which makes this kinda pointless. If they went down the supcom route with wreckages/reclaim, this would be far more viable, but with ubers current direction this wont work.
I like where you're going with this. What if the more metal storage you have the faster your units reclaim? (That is if reclaiming becomes more useful.)
I think the problem is more that wreckages have way not enough hit point than being able to destroy them.
Im not saying that the direction uber is going with wreckages is wrong, but it is certainly pushing down the opposite route of reclamation. Increasing the hitpoints wont fix the issue with reclamation =/