Following up with an answer to my own question...

Installing new front shocks and upper ball joints this past weekend, I loosened the torsion bars almost completely, basically unscrewing the rear anchor arm bolt from below until just a bit of thread was still exposed out of the upper nut. This allowed more movement in the control arms (I had already removed the sway bar earlier). I still can't answer the question of how much tension is on the bars once they are loosened and you still need to remove them from the front anchor, but simply loosening them to make front end maintence easier was straighforward...

The only difficulty was getting the ride height back to where it was after cranking the torsion bars back to their original position. I had measured how much thread was sticking out of the upper nut before loosening and reset the tension back to this point. But, the ride height seemed a bit higher than before, based on the distance between the top of the front tires and the top of the wheel well. Perhaps this is a reindexing issue, I don't know, its just took a little tuning to get things back to normal.