The source code is published on the GNU Savannah website (
http://savannah.gnu.org/projects/xboard ).
I also push a copy of most of my projects to my on-line git repository, at
http://hgm.nubati.net/cgi-bin/gitweb.cgi , which sometimes is more recent (or contains branches that are more recent), but not final.
We don't have any plans, but release if the code seems ready for it. Once every month we do a so-called developer release of the code in the master branch. Occasionally we split of a new fork, (most recently v4.7.x), which then will only accumulate bug fixes, but no new features. From these branches we do the official releases.
The front-ends are in the directories xaw, gtk and and winboard of the source tree. The winboard directory is still the old WinBoard front-end. The other two are based on the unified front end, which has most of the code moved to files menus.c, dialogs.c, draw.c (using the cairo graphics library; I don't know if that will have to be replaced by native Mac stuff) and usystem.c (for non-graphics unix stuff).
The front-end is based on the table-driven dialog creator GenericPopUp() in xoptions.c, which will be the main thing that has to be ported.