Agreed, the North Cascades is one of the most difficult areas to explore with the closest superchargers being in Burlington or Arlington.
I always charge up to 100% at Burlington when heading up. I camped overnight to get an early start at Cutthroat Pass and I had to stop for a bit at the free L2 charger in Newhalem to make sure I had enough to get back to Burlington. I didn't even use camp mode because I couldn't afford to lose anymore battery. At least the L2 is 11kw and I got a decent 37mi/hr but the last thing you want to do on your way home is sit at a slow charger.
Before I discovered the Newhalem chargers, I figured I'd have to head to Mazama or Winthrop for a destination charger to get back. This is why I think all NP should have chargers at their visitor centers or somewhere in the parks. Even if a supercharger is located just outside the park, it often doesn't leave much range to explore or camp overnight, especially when you add in elevation.
Newhalem seems like the logical place to build a supercharger but I have no idea how to request them. It's a tough one to justify though since SR20 is closed for half the year and likely wouldn't be worth the investment.
You can check out the planned supercharger map which shows there is a SuC planned for Okanogan in 2022 but ABRP says that even with a 100% charge at Burlington, you wouldn't make the 172mi. trip it to Okanogan. In other words, you wouldn't even be able to utilize it for crossing N.Cascades on SR20.