I have shifted the mailnews sync engine into a separate extension so that it can be used with existing weave extension. You can find the code here.
I am currently working on this repository but the basic engine functionality is already implemented. This extension should be used along with latest Firefox Sync version(1.4a1pre). New mailnews engine however will not show up in the Sync preferences pane unless a slight code change is made in weave(http://pastebin.mozilla.org/733149). However the mailnews engine is set to be active by default.
I will start submitting the mailnews engine patches to Weave as well soon. Since Weave is due to be a part of the central mozilla code i need to have the new engine code integrated into Weave.