Pressing the centre and return buttons while pressing reset definitely does something to the device where it forces it to boot from the SD card.
Simple guide below - I'll try to explain better, so if you have any questions let me know.
With your reader working correctly, all you should need to do is:
1. Plug your reader into your computer.
2. Download the skinMod script
and a skin (example penguins here
3. Unzip skinmod-1.1.zip, and copy backdoor.sh to your reader internal memory (not the sd card). If you're using Windows it will go on the root of D: or E: or F: etc depending on where your reader is mounted. If you're using a Unix variant and it mounts at /media/elonex then you would copy it to there.
4. Copy the skin (skin.penguins.0.9.tgz) to the same place that you copied backdoor.sh
5. Rename the skin to skin.tgz (case is important, if it can't find this file the reader will just boot up normally with no changes)
6. Disconnect the reader from your computer - you'll be back to the normal library view.
7. Turn off the reader normally by pressing the power button (no resetting or reflashing).
8. Wait a few moments, then turn the reader back on.
The device should start up normally, the led will flash while it extracts the skin file, and a skin.txt log file will be created on the internal memory.
It should be fairly obvious if it has worked - instead of the Cool-ER logo start-up screen it'll display a penguin.
If it hasn't worked, once the device has started up you can plug the reader back into your computer and look at the skin.txt file, which should give a reason why it didn't work. If you find any problems you can send me the skin.txt file and I'll help.
The script shouldn't be able to stop the device from functioning - the only thing I can think of that might do that is if you do the vulcan-nerve-pinch reset on it.