I don't have my n900 yet, but in the older maemo repositories there was a port of emacs and a maemotex package to supply basic TeX. Maybe these will become available for maemo 5 as well.

Your best bet would be easy debian, which is in extras-devel at the moment and should be available soon. With this you should be able to get native TeXlive and emacs working.

It's an interesting topic and definitely one I'm going to explore on my device.