Actually, Syllable, ReactOS, and Haiku won't have anything to do with linux. Haiku is a free reimplementation of BeOS, Syllable is a descendant of Atheos, and ReactOS is a free reimplementation of windows NT. (I'm not sure about the others, but i think SkyOS is proprietary.)
Codaeus --- please share your experience with any of these you try, especially any of the free ones.
I don't know if you be able to run them in vmbox. I think you'll be able to do that with Haiku, because they make some effort to allow this. (Haiku has the disadvantage, from my viewpoint, that it is not [yet] multi-user.)
And i'd be especially interested if you wrote any software for any of them ---- how much does the OS lend itself to that? Other questions i'd have about your experience would include how easy is it to cut and paste, whether they can export their file systems to other machines, and can other users remote log in in the background (graphical or not). (That turns out to be really useful in my experience, even if your primary goal is to write or use GUI software.)
Thanks in advance for any words you might have.