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I think Supernova lights only work with other Supernova lights; unlike most other dyno lights, their front lights also have the driver circuit (incl. standlight) for the rear. The easiest option is probably just to get a B+M rear light; their toplight line small is minimalist and bright with a standlight, and is available pretty cheaply, though if you're not rack-mounting it, you may need an support bracket.
B+M Cyo N+ (not sensing) front light, shimano XT 6w3w hub, Supernova E3 rear light.
Rear light doesn't have its own capacitor for stand light cabability, but front light does have one for the front light, but not for the rear light? Do B+M rear lights have their own cap, or can I rewire the B+M Cyo internally to give stand light to the rear as well. Or is there some plug and play 'add a capacitor' solution thats really obvious that I'm not thinking about?