I noticed that the --with-moonlight configuration option is provided in the 1.2.5.1 distribution of mono and thought although the .NET 2.1 support is currently in alpha, that having a USE flag to enable moonlight support would be beneficial for future moonlight-related ebuilds. Reproducible: Didn't try
Created attachment 143175 [details, diff] Patch to add moonlight USE flag to mono-1.2.6-r1.ebuild This is just a patch for the mono-1.2.6-r1.ebuild file which adds moonlight support to the ebuild. I used it to create a mono-1.2.6-r2.ebuild file which then installed mono properly with moonlight (.NET 2.1) enabled. This was on an x86 system.
Also, I want to make a note that I was mistaken in my first comment. The 1.2.5 branch of mono DOES include the --with-moonlight configure option, but not the required profile (net_2_1.make) in order to build it. The 1.2.6 branch however, does include the required profile and was able to build successfully.
Fixed in -r3, thanks!