I haven't read of this done before, but enclosed is a picture of 10 mega328P AVRs. The AVR is on the back side of the PCB in the photo (below). The PCB is Anarduino's "Miniwireless" PCB with a HopeRF RFM69HW data radio). Low cost.
They are wired such that one run of the downloader updates all at once.
Almost silly, but the handshake responses in the stk500-like protocol are sent by just one of the 10. The other 9 serial TX pins are not connected.
But the timing works since all run the same clock and bootloader code.
The RX and DTR pins are daisychained.
I'm using this to speed development, while I'm improving my bootloader which does over the air (OTA) wireless remote reprogramming of the mega328s. That bootloader now does one at a time OTA reprogramming but not gang programming. I'm adding gang wireless remote reprogramming - because it takes too long to do these sequentially (non-gang). The tricky part is dealing with modules that get an error on wireless.
Anyway, the wired version of the gang reprogramming is simple.
I'm using AVRdude as built-in to Atmel Studio 6.2. The JTAG/ISP is used only to install the bootloader one time.