I would still have expected the A1200 to show some signs of power (LEDs) - even with corrupt/blank roms (or even none at all) as this is just the 5v rail powering up, and has nothing to do with the ROM information being read.
Have you tried removing everything from the mainboard (floppy, hard drive ribbon/power cables, etc
Is the A1200 in the desktop case, or towerised - if in desktop make sure you have not bent the metal shielding somewhere which could be shorting the mainboard out.
As a test you could try using a PC power supply to supply power through the floppy power header on the motherboard.
