I don't know about the grommet. It seems like that would be a lot of force on it and might just push it out of the webbing. But I'm no expert on grommets.
I know that some of the earlier designs used rings to hold the spreader bars, so, yes, something like descender rings would work.
I sew pockets of wider webbing to the arch webbing to hold the spreader bars. I find that to be very easy and to need no knots or special equipment.
If you search the forums a bit you will find that a number of ways have been used to attach spreader bars.
Good luck with your design.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view. May your mountains rise into and above the clouds. - Ed Abbey