Currently the AI has simplified routines in certain areas, because it takes too much processing power to do full CEM for all the aircraft at the level it's done for player controlled ones. To stop the AI from cheating by flying a simplified FM there are artificial limits being put in place as we go along.
I think the easiest way to provide a solution without using more CPU cycles is to introduce a delay between engine start and take off for AI aircraft, maybe having it depend on the AI's skill level.
For example, a rookie AI would have a longer delay to simulate an inexperienced pilot warming up. A veteran or ace AI would have a shorter delay to simulate accelerated warm-ups by a more experienced pilot (i do this all the time, step on the brakes and advance throttle a bit until the engine starts to shake and then back off a bit, when the temperatures climb a little higher i advance throttle a little more, etc, this makes the warm up much faster).
This would not have to be a complete CEM simulation that takes away CPU power. We could even add a bit of wiggle room to randomize things, also dependent upon AI experience levels to simulate that experienced pilots would be more consistent in their handling than inexperienced ones. It would just be an AI routine that goes like this:
IF AI_level=rookie then delay=X +/- 10% seconds
IF AI_level=veteran then delay=Y +/- 7% seconds
IF AI_level=ace then delay=Z +/- 5% seconds
Where X>Y>Z
etc etc
On another note, i made a sample Blenheim bombing mission to test out the FMB, setting the 4th aircraft in the take off row as player controlled. The aircraft in front of me do start up and take off immediately, but they don't follow the course until the rest of us are up in the air.
While i'm warming up (and delaying the take off of the AI behind me) the first three aircraft are circling the field waiting for the rest of the flight. I didn't have to provide waypoints for this or do something special, it just seems to be default AI behaviour to loiter over the field until the entire flight is up in the air, so their instant warm-up is not that big a problem.