It sounds like it might be possible to do with the console. I mean wouldn't you be able to flag the main quest as completed like any other quest? I haven't ever done it myself so I don't know what you'd enter to do it, but it'd probably be easy to find on the UESP wiki.
EDIT: Maybe "setstage MQ16 100" ? Just scrolled down to the bottom of http://www.uesp.net/wiki/Oblivion:Light_the_Dragonfires. I'm not even halfway through the main quest so I have to avoid spoilers, hehe. Not that the name of the quest gives anything away :whistling:
the sacrifice you make getting so crazy into modding before getting very far in your glacially slow initial playthrough