I don't have progressive / variable regen, but I can say that the on/off EABS (stronger than just regen, as it actively brakes the motor, and goes down to nearly zero RPM where regen doesn't always go below a few MPH), on my SB Cruiser's left wheel (regular regen on the right, presently) brakes really hard--if it were on a regular bicycle, it'd probably skid the wheel (the trike is too heavy back there for that to happen; if it were on the front wheel it might skid it).
If it were variable, it'd be a great brake. Since it's not, there are situations where I have to use the regular front brake to slow a bit first and then use the EABS + regen (left and right are linked),
So I'd guess you'll like variable regen (or better, EABS if you can get it), as long as you can set it strong enough.
Mine is just a generic ebike controller, nothing special, so I'm sure there is better stuff out there with good braking setup. If you're willing to build a controller and power stage, you could look into the Lebowski stuff. (I've been collecting the parts to build a pair for a while, partly just to get variable strong braking).
A few ways to simulate a load on the lever, that I've considered:
--You could use a cable from the lever to a spring load.
--Or to a brake pad on a pivot pressing against the frame instead of a rim. (the spring load would probably be easier to make small).
--Or directly use a spring (leaf instead of coil) to the lever so it has to bend it as you pull.
--Or a pull-coil-spring from the lever out the cable opening to a mount on the bars or stem (or something else mounted on the lever housing, if you can find a spring narrow enough for that that still has the tension you need.
--simplest I can think of is an elastomer block under the lever (on the bars/grip) that squishes as you brake, but I'm not sure what would be compressible to the right extent.