Starting a sub-mission (Taxi, Firefighter, Paramedic and Vigilante) right after savewarping seems to not change the on_mission flag value. This allows savewarping in those missions, similar to the method described in http://www.speedrun.com/gtavc/thread/z83eh, but involving no rampages. The timing is precise from what I've tested but you can try again right away. Practical applications of this glitch would be to use savewarps during Taxi (not Paramedic, as health does not get replenished after every level), or, if it works without crashing, duping Taxi fares, as seen in the video below.

edit: Duping taxi fares work. I saved after 98 fares (not sure if the save is needed but I did it for safety) and did the last two normally. Took just 67 minutes, but could easily be 55 or less with good execution and RNG.

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