As for third party, it's not really possible with the way Unity exports the game, you need to have access to the original files to make another version. I have never tried to export a decent sized game, only minor tech demos to test menus, but there was not a lot to it, other than having to export it once per platform. It could be that the UI doesn't work well without tweaking, or it could be that demand by the supporters isn't high enough, or some other reason, I couldn't say. Maybe I will be able to let you know once I get far enough along to try it myself.
Since this questions pops up so much, here's a short explanation:
Unity makes it very easy to export to other platforms.
The reason I have not done so for Mac, is because I don't want to maintain 4 versions.
Since I update every 2 weeks and it takes a lot of time as it is, I don't want to complicate it any further.
On release I will export for Mac as well.
For Android, the same basically applies.
I have made mobile games in the past and exporting them for both Android (and even iOS) is made very easy by Unity.
However, MotH was never designed with the intention of running on mobile devices.
So while I could easily do an Android export, without a LOT of changes to, mainly the UI, it would be pointless as it just would run very poorly, or not at all (think of UI scaling, all the mouse over effects, etc.).
So this is not a limitation of Unity at all (in fact it's one of Unity's benefits), but one of MotH itself.
Hope that clears things up a bit.
