"Unfortunately, cheap, off-the-shelf WiFi repeater units are not available and the network configuration for the devices we did find was extremely difficult, so

"Unfortunately, cheap, off-the-shelf WiFi repeater units are not available and the network configuration for the devices we did find was extremely difficult, so we decided to use consumer-grade Linksys routers loaded with the open-source dd-wrt firmware." Did you look at Ubiquiti devices? Their PicoStation would be perfect for this. Also, and this depends on what you consider cheap, I would have used 900Mhz for the connections between repeaters, base station, and robot since it will go further through rubble. Use HWMP for mesh across the repeater links & use 2.4Ghz "access" at each repeater for access/interop with other devices. This could be done for under $500 per repeater with Mikrotik router boards and Ubiquiti mPCI cards in 900Mhz & 2.4Ghz.