I think boons on a Defense Roll are a great idea. However, there's no TN to base a Boon off of either. I'd use the standard DEF TN (15+SPD) as the TN for Boons. So, the base roll would be 5+2d10 vs. TN 15, and a a result of TN25 would score a Boon point. Remember the average roll is 5+11=16, so you essentially have to roll explosive on a d10 to even have a chance at a Boon.
Because its a zero AP roll I would be careful on allowing any actions as a result. Bonuses and movement advantages on IMO; no, disarms etc. For my 2 cents.