Great. Sounds like you know more about ppp than I do.
What baffles me is that for the USCC (US Cellular) people (or at least some of them), enabling authentication in the RIL caused their connections to start working... without them actually having to enter any authentication information. Nothing in pap-secrets or chap-secrets, nothing in the APN (I'm not sure the ppp agent can read user/pass from the APN yet, so that might be moot). Clearly, whatever's going on, I don't know what it is.
OK, new, slightly less boneheaded idea. Old rootfs (with no data) -> Start up, put the phone in airplane mode. New rootfs (that's been causing reboots) -> Start up, hopefully the airplane mode keeps it from immediately rebooting, put your login info in pap-secrets, chap-secrets, and options.smd1 as instructed in the linked thread, turn off airplane mode, and hope it doesn't go back to rebooting.