I need to hang a thai bag in my garage. I was planning on boxing in a 3x12 piece of head board in between the trusses. In doing so, is it safe to use 1/2" lag bolts throughh the truss?

I would run a 4 X 4 between the joists as it would allow for more holding power for a lag bolt.

You can always use two 2 X 4's flat,across the opening also,allowing for the bolt to go through both 2X's.

With the amount of force being put to a bag,a single 2X12 wouldn't have enough thickness to it to hold(IMHO)