if it works within your final plan, i suggest the following:
1. no matter what, stick to your plan of perpendicular to the trusses. this is good for more reasons than most realize, particularly once you start kipping,
2. mount the bar not to the trusses, but to a 2x6 that's mounted to the trusses, this way, your bar has no breaks along it's length, but is braced the whole way across,
3. screw through the board into each truss twice (if you use a 2x6, then about 1.5-2 inches from the side). this will keep it more solid, particularly when kipping
4. washers are your friend. not because you care about the cosmetics of your trusses, but because they will help distribute the load across an area greater than the head of the lag bolt.
i did all these, and mine holds a minimum of 400 pounds even waking up.
good luck man. we wanna see pics when you're done.