Thanks for the tip, Paul.
I went ahead and lubed it up before trying to seat the grommet but was still having issues. Very obvious tip to anyone else that does this job: once you get the cable routed and grommet started in the firewall hole, just go back into the vehicle and pull on the cable and the grommet will pop into position with just a quick pull.
I spent probably over an hour trying to push the grommet in, cutting up my arm in the engine bay in the process, before the "hey, you idiot" thought came across my mind.