Yes I guess you could but then you need to turn the engine over to install the rear main seal. Just a whole bunch easier to do with the engine out. You have to stuff special felt seals soaked in wellseal down the side of the rear main seal. I'm not sure that you can get the bottom half of the seal out with the crank in the engine. You have to pull the oil pan to get at the rear main seal. If your engine runs well why tinker with the rear main seal?
Lot of different thoughts in that paragraph but the upshot of it all that the rear main seal is a son of a gun to install, you have to have an allignment tool and get it all alligned without the crank in it. So I guess I'm back to my original thought, that you can't do it with the engine in the car.
Good luck, now that I have totally confused you, heh. If you would like to discuss this further PM me.
Regards, Tinkerman