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Kinetics can do this but you will have limited choice on gearing .. single or alfine/rohloff
There's no reason you couldn't put a belt pulley on a three speed internal (Sturmey/Sachs/BSR/BWR) hub as they are available in all fitments.
Putting a splitter into a rear triangle shouldn't be too much hassle, getting a splitter might be. Ceeway doesn't carry loads of options. If Paragon do them then Peter can get you one in his next order or getting one custom made would be an option.
Probably going to need a custom tensioner/snubber or whatever they call them when it's for a belt and you might need to compromise on gearing to get a combination that'll get to the right tension as you unfold.
Anyone converted their Brompton to a belt drive?