Yesterday, official support for MFC/R2 signaling in chan_dahdi was merged, and will be included in Asterisk 1.6.2. Here is the commit.
To get a head start on testing Asterisk 1.6.2 with R2 support, check out the code from the 1.6.2 branch:
$ svn co http://svn.digium.com/svn/asterisk/branches/1.6.2 asterisk-1.6.2
Before compiling Asterisk, you will need to compile and install LibOpenR2.
Thank you very much to Moises Silva (Moy), who worked very hard to write LibOpenR2 and bring this functionality to Asterisk!