I believe the whole genius of Demonreach is that it CAN contain the otherwise uncontainable. But, Angels do operate on a special level, across multiverse, and I have my doubts Demonreach could contain that.
One possibility I see is that it will serve as a special timeout. As in, Kemmler was Warden once, maybe he found a stash of 5 coins and set them free, and if Harry stored coins in Demonreach, his successor would probably inadvertently throw them out somehow. We know timeouts apply to the Fallen, and to an eternal being, what is a decade or 5? It'd suck if it's apocalypse-now, but it still is just the once.
And in that sense, I think the lack of blood is indeed important - if Harry can't stash them in crystal, any Warden could easily pick them up, no skill required. Though we know a Warden could even release those properly trapped, it seems harder.