I have installed Regenerative braking on a Mid drive (which slows down my Rear wheel), but on the lever of the Front brake, and I realized the following
On a motorcycle, and a bike in general, we use the Front brake most of the time because if you rear brake hard the wheel can lock and slide. By using the front brake, as you brake the front wheel presses down harder on the ground, which increases friction and makes the braking more efficient.
But what I found after installing rear regen on my front brake lever, is that as I go to use the front brake, as soon as I touch the brake, I get a deep slow down of the rear wheel, but not harsh enough to lock it. As a result, I almost never have to engage the mechanical front brake by pulling the lever further. I only have to do that on the most intense braking situations.
As a result, I would see very little added benefit on a separate regen on the front wheel, and a lot of drawbacks in terms of weight and complexity.