Anyhow I recently discovered something odd:

If you strafe DIAGONALLY after running out of the tunnel, the Obelisk will not fire at you! Six times in 2 different games I ran out of the tunnel, firing grenades at the obelisk and strafing diagonally (holding both forward and straft left keys). I was able to hit the obelisk 4 times and then I was close enough to it (between it and the HoN) that it couldnt hit me, so I ran inside and planted timed c4. Since my previous attacks always resulted in it zapping me, no engineers even bothered to come check it out. My timed wasnt enough to destroy it, but I did cap 2 guys that spawned in there.

In the second game I would then run out of the obby and either steal a vehicle, or attack the airstrip