Well, the thing is that in the lab, you're directly changing the memory - FS2 doesn't save TBL, POF, or FS2 files though, so changes only last as long as FS2 is open.
It's possible to override stuff with SEXPs, but that'd take much longer; this is mostly for "I'm working on a mission and want to play God to see what would happen if........"
For example, if a set-ship-velocity SEXP was implemented. (which I intend to do sometime soon, now that I know it works how I thought it did.) You want to have a ship jump in from hyperspace. So you make a mission with the ship some kilometers away, where it isn't really visible, set the speed to something ridiculous (20000 m/s). Nope, that didn't work, it was too short. You reset the position/speed and try again, until it works properly. You write the final distance and speed down, and use that to make the ship 'jump' in in future missions.