Colin,
Your flyer base hinges in the front right?

I always tie the trap lines to the beam and it provides a hinge so the mast can only move front to back, not side to side. Remember to put the pin in the mast base.

Stick your wheels or something under the bows so that the mast is leaning back and the shrouds are not carrying any of the weight. Bows should be into the wind, so the combination of wind pressure and gravity should keep the mast from moving.

Undo the shrouds, and then lower the mast.

You could always rig a line with a slipnot that you could undo from the mast base, so you wouldnt have to leave the mast unattended, or you could get a stranger to pull the pins on the shrouds. Might be a good way to meet girls, "Excuse me, but could you pull on my pin? I mean...no wait come back...."