It can't work anyway
To be fair I said this early on. I'm not a programmer (more a tweaker if that). I'm assuming you are as you seem to have knowledge in this area. I still think this can work.... Just not at this point in time. But I do believe someone will figure out the signing keys eventually.
There's still other problems though. The switch hasn't been software exploitable since V4 fw. So it'd potentially mean downgrading the FW, then the switch wouldn't run due to a fuse count. And yes I know the hyperthetical Nsp file wouldn't do anything the browser hack didn't (just more convenient).
Is there a way to boot to this CFW without keeping attaching my switch to the PC and loading the payload fuse?
I bet you've been following this laughing. But the easiest way for you would be via RCM loader. There small enough to carry in your switch case and eliminate the need for a laptop.