It's going to be tough to make a nice looking AV install using 2 post racks.  Do you have any equipment that needs support from rear rails like rack mount UPS?

If the client wants a nice clean looking installation you might try convincing him to replace the racks with something that's more appropriate for AV gear.

Another idea would be to add more of the 2 post racks behind the existing ones so you have rear support rails for attaching equipment and traditional lace bars.  You could even get creative and make a side panel on the end rack to hide the wiring.

I didn't really understand your description of the layout... if the racks are only 3ft away from the wall, it's going to get tight accessing from behind.