For some reason, I thought the 68K processor was 16-bit, so I told lwip a pointer uses 16 bits in the cc.h header file. (It has a 16 bit data bus, 24-bit external address bus, but is 32 bits internally.) Now, you would think I would have figured this out after using 32-bit addressing to get past the previous link error. Or, maybe I would have looked at the lwip cc.h for the coldfire processor, which is a 68K at heart. Or, maybe I would have read the Apple SC/SCpp Reference more thoroughly the first time around. Anyway, MPW for 68K uses 32 bit pointers.
Quick fix to the code:
typedef u32_t mem_ptr_t;
And... no assertion errors at run-time! It still isn't completing the PPP negotiation, but it is trying so hard: PowerMac-G4 pppd <notice>: pppd 2.4.2 (Apple version 233-0-8) started by epooch, uid 0 PowerMac-G4 pppd <notice>: Connect: ppp0 <--> /dev/cu.usbserial PowerMac-G4 pppd <error>: Received bad configure-ack: 02 06 00 00 00 00 05 06 bf 68 ca 18 07 02 08 02 PowerMac-G4 pppd <warning>: IPCP: timeout sending Config-Requests; PowerMac-G4 pppd <warning>: acscp: timeout sending Config-Requests; PowerMac-G4 pppd <notice>: Connection terminated.