Cmon people these things aren't built with the bootloader on them.
Think man, think!
When you try the vol down + power to get to bootloader does the screen show anything?
I tried this on my N7 16gb and here is what happened: Booted to fastboot, issued "fastboot erase bootloader", successful no other selections, power + vol up did nothing. Powered down, used vol down + power, got into bootloader screen showing the <START> option only. Fastboot devices did see my N7, issued fastboot reboot-bootloader and came back with all the options. Tried this again, same results. Then I booted to recovery, reflashed my bootloader zip file with CWM and the bootloader was normal with the hardware buttons again.
Can't say why this worked, but went through the whole process twice and it did.
I don't recommend anyone try this though, maybe I just got lucky or something?
EDIT: some other info. When bootloader had the <START> option only, pressing power on that option DID boot the device, so maybe my bootloader wasn't completed erased? Also, I'm on JB 4.1.2 but I'm using grouper bootloader 4.1.3 from the JB 4.2 build.