Has anyone worked on porting Allegro to the Wii for homebrew? Version 4.2, 4.3, or 4.9? I'd love to port my current project to that platform and really don't feel like recoding the necessary parts into SDL.

Can you do OpenGL on a Wii?
If so, it's probably fairly straightforward to port A5 to it. Not aware that anyone has tried to do that though.

Going from 4.2/4.4 to 4.9/5.0, you'd have to do that anyway.

I know that there would be some rewrite required to move from 4.2 to 4.9, but it would be less than rewriting from Allegro to SDL, right? Going from 4.2 to 4.9 would just require some tweaking here and there, or is Allegro completely incompatible with 4.2 code?

Doubtful.
Don't know SDL though, so I can't say with 100% certainty, but Allegro 4.9 is completely different from 4.2.

Yes, completely.
Peter Wang used a partial wrapper around A5 to port speed, so that emulates some of the more common function calls. Eventually, we should probably "port" A4 to A5, but we don't have that yet (and likely won't until 5.0 comes out).

Given a sufficiently abstracted interface in your own code (only do direct calls to Allegro in a single module) its not that hard to port from Allegro to SDL. I ported my game to SDL in about 2 weeks.

I also compiled my game using the SDL wii port to successfully run it on the Wii. It is most likely much less work to port your game to SDL than to try to write an Wii backend for Allegro5.

SDL Wii stuff: http://wiibrew.org/wiki/SDL_Wii

I set up the whole wii cross compilation environment for linux (with SDL), with libz and libpng compiled as well. You are welcome to grab it:
https://paintown.svn.sourceforge.net/svnroot/paintown/build-environments/devkitpro