Note to self: if you happen to write code that AVs the FJIT in Rotor, this prevents you from building the FX tree. I honestly don't quite know why the runtime is loaded there and what managed code executes, but according to Jan Kotas, a dev on the CLR team, it is. And furthermore, the debugging experience kind of sucked... the build log had no errors in it, but binplace failed to find the output DLL when it tried to copy. Sure enough it didn't get built. I'm pasting the error for folks searching in the future:

Binplacing - objd\rotor_x86\system.xml.dll for all platforms
binplace : warning BNP0000: CopyFile(C:\dev\play\sscli-1.0__STM\fx\src\xml\objd\rotor_x86\System.Xml.dll,C:\dev\play\sscli-1.0__STM\build\v1.x86chk.rotor\.\System.Xml.dll) failed 2
binplace : error BNP0000: Unable to place file objd\rotor_x86\System.Xml.dll - exiting.

It killed at least 1 hour of my time tracking it down. Running a quick test under the devenv debugger:

C:\dev\play\sscli-1.0__STM\tests\il_bvt\base\objd>devenv /debugexe %TARGETCOMPLUS%\clix.exe ceq.exe

Did the trick. Stupid bug, easily fixed, and now I'm back building the tree again. Hoorah.