Mechanically, the rod should not be able to come out of the M/C, grommet/o-ring, or not. I cannot physically move the brake pedal rearward enough to make the rod fall out. Metal/brackets under the dash stop the pedal before the rod is even close to falling out, plus as DD says, even with an o-ring in there, you'd have to pull like hell to pop it out in the 1st place.
Also, highly recommend speed bleeders. No wife, girlfriend, or other, needed for assistance!