Serial programming of several AVR microcontrollers simultaneously via ISP interface
The AVR ISP programing protocol both writes to and reads from the device being programmed during programming. For this reason you will not be able to do this (program multiple devices simultaneously) unless you write your own software to do writes only (and take the significant risk that you miss a dead device on your panel). http://www.atmel.com/webdoc/protocoldocs/protocoldocs.avrispprotocol.html

There are many commercial gang programmers on the market (though you'd expect $2-3k pricing), but given the ridiculously low cost of an AVR ISP MKII programmers on Ebay, you could hook up multiple USB programmers and cycle through the various ports. Not exactly gang programming, but certainly could be faster than a manual single device process. If you are really keen to save time, then you could roll your own software to gang program multiple AVR ISP programmers, it would not be difficult.