This has come up before.
It would only work with controllers that support that form of braking.
Most controllers are just on/off braking, either regen or EABS.
I don't think there aren't any commonly available levers that do what you want, but you could make one using parts from a throttle, either resistive or hall sensor type. Might need a different magnet set depending on the lever travel vs the sensor placement. The simplest mechanically is to just use a cable brake lever, and use that to pull a cable in a mechanical box somewhere else, in the controller box, etc., or to have it pull a throttle that's connected to the proportional regen input.
There's some threads about doing that, too, though they were used to make weatherproof throttles they'd work for this, too. I think GWhy was even making them for sale.