If you've already got a hub motor, and you've got electronics skills, a power resistor (rated to perhaps 100W) per phase on a heatsink (perhaps under the downtube) should do. You want the heat dissipated in moving air, rather in the windings, which will determine the resistance you can use. The difficult bit is likely to be getting motor specs in enough detail to avoid too much trial and error.
To switch it in, at least initially, I'd use relays (or one relay if you can find one with enough contacts. That would make it easy to disconnect the existing drive circuit at the same time, using changeover contacts. This would act as a drag brake with a fixed relationship to speed, so I'd start it at the top before building up much speed, otherwise the turn-on would be rather sudden.
I'd probably also add a little digital thermometer to the resistors - you can get cheap battery ones designed for panel mounting, with the sensor already fitted. This could be removed after testing, but I'd want to know if I was likely to melt the solder and lose contact, easily done with big load resistors unless they've got excellent heatsinking.
With well-chosen resistors, it should be possible to make this an effective drag brake to keep your speed down, but almost useless for stopping you.
There are also active load braking circuits, that dump the heat into MOSFET motor drivers (again with big heatsinks). I haven't used them myself in this application. They'd need more of a control circuit, but could be used to slow you down gently once you were going fast. However packaged versions may not be designed to provide long-term braking. Looking at the datasheets on these may give you a hint as to resistances to use.