Already posted at gtaforums.com, but I leave it here too. (Also, I hope T2 lawyers choke to death)
Found a "cheaty" workaround.
Use some trainer or save anywhere script (I've done it with RZL-Trainer).
As soon as you have killed all couriers, when you are returning to
Zero's roof, "preselect" the Save Game option, it is redded out, saying
"On Mission", but keep it ready. When the Mission Passed text appears,
the trainer menu dissapears, but it is still there.
So now just mash the selection key, until the mission really ends and
the Save Game option we have preselected just unlocks, so the Save Menu
just appears in front of us.
Save, suffer the crash again, load. PROFIT!!
(Sometimes the game gets really "crashy" when you save and load that
way, so head to a proper savepoint or safehouse immediately)