Pulling the cable head through the firewall and pouring lubricant through is as good a way as any to do it. I have always used graphite in carrier solution, as this persists better than grease and seems to work at least as well. One can get this product as an aerosol-there used to be an oilcan type dispenser, but haven't seen that of late.
This works well for vent cables etc as well. For the smaller ones where room to insert the lubricant is limited, a hypo needle works very well.