Maliit developer here. There are two VKB plugins provided by us:
* QML-based: very basic, but works on more platforms than Harmattan.
* Harmattan: the one that gets all the nice reviews lately.

We would like to improve the QML version, but at least for the N900, it is feasible to get the Harmattan version to run (its dependencies are all available for N900).

The problem with Maliit on N900: We have only basic support for Hildon-based apps, and that's sadly the majority of apps still :-( We have a GTK+ plugin, but it needs more work.

So even if we don't have much time for the N900, we do accept patches. In fact, I just merged a N900-specific patch on Friday ;-)