I would never let there be a time when you're handling your snake and the cat is within reach of its head. Just like if you were holding it near another person; all you have to do is arrange things so that the snake is not going to bite someone's body, be that someone cat, human, wombat, anything.
I have had one of my BPs grab/comstrict my hand.. granted, it was not a boa like you're discussing, but I think a boa would be more likely to strike out b/c they hunt by motion.. caution is all that's really needed.
The only time my cat creates tension, is if he walks by my boa's cage while I am trying to feed the snake; the motion can attract his attention and take his eyes off of the intended prey target; I just yell at the cat and he is gone.