You may want to consider a hardware fix vs software. One option would be to hack a cable (you can get them pretty cheep after all) to short out the data lines. That will imply to the N900 that it's a "dumb power supply", and will usually trigger it to move to 500mA mode. The charger, as I recall, has a fixed level resistor in place to indicate that it's 1A capable. Doing a search for that should reveal a post or two about it.

This eliminates the need for software hacks and/or changes that may damage the N900. Worst case you hose a cheep cable.