Now I know there are alternative browsers for the N900 (Fennec for one) but those tend to either suck or be based on fairly old versions of their rendering engines.

My idea is to:
1.Find the version of mainline Gecko that matches with the Gecko version being shipped on the N900 (in the microb-engine package)
2.Having found this, identify any patches on top of mainline in the N900 Gecko version (whether they be patches that ended up in mainline Gecko later or whether they were Nokia-specific or otherwise dropped
3.Figure out which of the patches from #2 are actually necessary and forward port those to a more modern Gecko version
4.Make the other changes to the modern Gecko required to get it to function as a replacement for microb-engine and make it work as a browser
5.Reverse engineer the pieces that talk to the browser (flash player, maps, a couple others) and see if they are tied to the browser's APIs in ways that will break with a newer Gecko
and 6.Using reverse engieering and whatever else, find a way to make the stuff in #5 work.