More technical people will chime in but I think you can drive it while it is leaking - just keep an eye on the oil level. The rear main seal keeps the engine oil in the lower part of the engine and I don't think it should affect the transmission. Working on the head has nothing to do with the rear main seal (I think). Mine is currenlty leaking a little but it has not increased in almost 30,000 miles.
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (194,000)
1999 E300 Turbodiesel (244,000 miles, dies at 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles)
2011 Audi A4 Avant (129,000 miles) hate this car!
BMW 330 xi 6 speed manual (100,034 miles)