I can confirm that on Windows (mine Windows 7), you can use Notepad to edit the file. I don't know why, but when I used Wordpad it didn't work. Possibly something to do with formatting loss when saving.
I made the following modifications:
Next Page = set on 'Enter' Button,
Previous Page = set on 'Delete' Button
these work great and I left the Centre Button to do what the Kindle Firmware wants it to do.
I also realised that you can map the 'Alt' Button, but to turn a page you have to press 'Alt' and then 'Shift' afterwards everytime.