There are two steps to processing keys. Firstly XBMC has to detect and process the keypress, and then if XBMC accepts the keypress it uses the keymap.xml to work out what to do with it. My mods to XBMC work at the first step. It detects the keypresses sent by the remote and converts them to the standard keypresses used by XBMC. For example pressing the Stop button sends control-shift-S. My mods detect this and convert it to "X", which is the standard XBMC keypress for "Stop".
The keymap.xml will probably be enhanced for the next version of XBMC, but for Camelot the keymap.xml is fairly basic and can only process keys like "X", and in particular it can't process control and shift keys. That's why I have to intercept keypresses like control-shift-S, because the keymap isn't capable of handling key combinations like this.
So there isn't a good tutorial on the keymap.xml because it's so basic that anything that isn't obvious probably can't be done.
To get the Back key working the simplest course is probably to scrap your HP remote and get a standard MS remote or one of the many clones. The Info and MCE buttons probably send what we programme dudes call a WM_SYSKEYDOWN message. I haven't looked into processing these messages. It might be possible, but I suspect it would take significantly more work than the changes I've already made.
When Camelot is released I'll have another look at the keymapping and see what I can do. At the moment I have to redo my changes every time a new alpha or beta version is released, so I'm reluctant to invest too much effort in it. Once we have a stable officially released version I can spend more time on it.
As I mentioned above, the version after Camelot is likely to have an enhanced keymap that can process control and shift keypresses, so the changes I've been making will no longer be necessary.
JR
PS you might want to look at
http://forum.xbmc.org/showthread.php?tid=61571
This describes how to modify the keypresses sent by the remote. It's still on my list of things to play with so I can't say how well it works, but if it isn't too intimidating you might be able to map the "Back" key on your remote back to the Backspace keypress.