Idle alternator power on a bike is a bit not that important, it will be higher when moving. You still need power for ECU, ignition, fuel pump and injectors command, and this is basically what idle power is about.
To be honest, 75W heating is MASSIVE when in a jacket. 10 to 20 ? Yes, that should be ok. More ... not really.
I used to only drive a bike and no car for years, so i drove in lots of weather, including snow and sub zero tempertures.
I tried various stuff and really, for long trips, the stuff that goes on the handle bar around the grips and lever is the best you can do for the hands. It is called "manchon" in french, no idea how it is called in english. They are mostly seen on scooters, and now exist with heating too. Nothing could beat that for the hands when they had no heating, so i guess with it, it will be perfect. I tried heated grips, which is nice but does not heat the hand part exposed to wind, heated gloves which usually do not heat fingers either
For the torso, a couple of 5 to 10W pads is enough. It needs to be under insulation, not on top of it, so not in the jacket. In the cold you need clothes made to keep the heat. Equiping many heating pads without much insulation does not really work, it will burn locally and freeze everywhere else. This will clearly happen if you fit 75W worth of pads on a jacket. There really is no point doing this, you want the heat pads much closer to your skin.
Battery powered soles with overshoes to cut out wind and overpants to cut both wind and humidity are very good choices. You could fit battery powered heating pads on your thighs, i never tested that because my bikes used to heat there enough or have fairings protecting that area enough, but i think on a RE650, that could be welcome.
You are going to need some batteries, and pouches to store them on yourself though. But it is worth it: you will still be warm while refueling
Last thing, I test fitted an aftermarket car seat heater at home, on my gaming chair. It has 2 heat modes, 12W and 24W. I cannot even sit on the chair at 24W... so i'd rather not be in a 75W jacket.