That is very interesting, I never thought about that (probably because I rarely play with TAGs

). Ok, so if I get this right you actually can do that. Here is quote from the wiki:
ECM
"ECM has a standard level of 5, requiring a Normal Roll of 5 or less for each incoming guided projectiles to deflect and detonate the projectile without causing any damage.
ECM functions automatically and does not require spending of any Orders or having LoF and can be used in ARO. ECM allows reacting in ARO against enemy guided projectiles."
Hacking Guided Ammunition
"To hack a Guided projectile effectively, the miniature must make a hacking roll, a Normal Roll of
WIP modified by the target’s
BTS. Guided projectiles have a
BTS of –3 and only Guided Ammunition attacks may be hacked. A defending hacker does not need
LoF or to be within 8 inches of the projectile:
When a Guided Ammunition attack is made, any hackers on the defending side gain an ARO that can be used to hack it. If the hacking roll is successful, the enemy’s projectile is completely deactivated. Hacking Guided Ammunition is a Short Skill."
Also this:
FAQ:Hacking Guided Ammunition
"-Can I hack guided ammunition with a roll of each one of my n hackers? ( two hackers, 2 rolls, if I have N hackers , N rolls)
You can indeed react with each of your Hackers. this mean that you can do N rolls, depending on the number of hackers that you own."
So you can defensive hack with one or more hackers if you have them and than you can try ECM. That is actually awesome.