Thanks, pito

Manually added the boards.txt content for Maniacbug Mighty 1284P to the main boards.txt

Also moved the cores from it into

C:\Program Files\arduino-1.5.2\hardware\arduino\avr\cores\mighty1284P_standard

and the variants form it into

C:\Program Files\arduino-1.5.2\hardware\arduino\avr\variants\mighty1284P_avr_developers

C:\Program Files\arduino-1.5.2\hardware\arduino\avr\variants\mighty1284P_bobuino

C:\Program Files\arduino-1.5.2\hardware\arduino\avr\variants\mighty1284P_standard

I can't help wondering if there's a cleaner way of doing it, so it's easier to keep upgrading the Arduino environment, but that seems fine for now - the different types of 1284P board are now on my Board menu.