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Biggest concern is the port in those batteries, if all the batteries have those micro USB port, then there's risk of water getting in from that and make the system go loopy (think of those rechargable light that often gone wonky when water is directed at it without mudguard, Moon Comet for example).
If however, those little black bit are removable battery and can be charged like a cochlear aid batteries, that would be a much better system.
Edco were manufacturing a 11 speed monoblock cassette in CrMo hardened material to use on 10 speed free wheels for about £150 ish. I can't find it on there site now though ..