I would caution against using fabric in close proximity to lighting. Even LED fixtures generate heat. A lightweight flexible material like vinyl might work, but won't have much structure.
If you're goal is to preserve the grid for future reuse in the post-layout era, the trick will be a sturdy enough attachment so you don't have stuff falling down all the time. If you're not worried about "organic" flow to the valance, you could use aluminum coil stock cut which is available in various widths up to 24", cut panels to length and use a metal break to put an "L" at the top that can nest into the panel rails. Perhaps you could pop rivet that to the grid, then use separate pieces to roll curved corners that rivet to the straight runs...
I hope you can see what I'm saying... It could work, as long as the geometry of the grid above works out with your layout footprint to some degree.
Lee
