Exhaling at rest (unless done against resistance) is mostly passive and requires very little muscle strength.

http://oac.med.jhmi.edu/res_phys/Enc...lesOfResp.HTML

If you are having trouble exhaling to that extent, I would think that you also have some other problem (such as asthma) which narrows your trachea or bronchi. This could also make your MG worse (as your muscles have to work harder to breath). Did any one look at that?

Also, the way to decrease CO2 is by breathing as fast and deep as possible. Not a maneuver I would suggest for an MG patient with an impending crisis.