It's fully legal to replace the keyboard, assuming it doesn't have Nokia code, question is as hawaii said, will they accept it? It's replacing a core system component, which if broken, can lead to bad things.

(But then again, there's swype on Symbian, but that's tested by Nokia...).

And as for the "I have closed source components on my device, one more wouldn't matter" suggestion, we're trying to reverse that, not help make the device more closed.

Keep in mind that lots of open source projects are a lot bigger than a keyboard, have thousands of lines of code, and don't ask for donations

Edit: and here's an example on one that recently got a donate button in the about dialog, but nothing more:
[sbox-FREMANTLE_ARMEL: ~/columbus-0.9.3/src] > cat `find` 2>/dev/null | wc -l
5020