It would be extremely useful to be able to throttle the number of concurrent builds of matrix configurations that can take place on a given node. The reason for this is when you have a matrix project in which all subprojects share the same parent workspace data. It is possible then to run concurrent builds across separate nodes, but not on the same node.
The specific use case I am running into is for building Android. I have a matrix project that defines configurations for each device I wish to build. Every sub-project shares the same android source tree to build off of, so concurrent builds are not possible on the same node as they would clobber each other, but concurrent builds on separate nodes would be fine.