There is a way of modifying a micro usb plug/cable with a resistor that will force the phone into download mode even if the buttons no longer respond http://forum.xda-developers.com/showthread.php?t=819551
Cheers for the link something I can still try, though it says:
My phone don't seem to meet this criteria? Not even sure what bit I will have mucked up, I applied an update.zip that was meant to fix the 3-button problem, but I must have picked the wrong one. Do you know what part/level this tries to change?If the phone's bootloader is still intact, that is you see at least the white Samsung logo or the "PC ! Phone" logo on the screen, you should be able to force it into download mode by connecting a homemade jig that simulates the Samsung "Anyway" field test jig.
cheers