Derivations with "preferLocalBuild = true" can now be executed on specific machines (typically localhost) by setting the mandary system features field to include "local". For example:
localhost x86_64-linux,i686-linux - 10 100 - local
says that "localhost" can only do builds with "preferLocalBuild = true". The speed factor of 100 will make the machine almost always win over other machines.
SK6WHODMWAUL7LNH6QTUXZW7GQFNKKDYN4BU45ODTMMKYMG6O7KAC