It may not be a very helpful work around for you, but I found with my experiments that I was able to start a concurrent build and the git plugin detected its changes, so long as I launched the concurrent build myself, rather than relying on SCM polling to detect the change.
Since there are command line calls and wget calls which can launch a concurrent build, maybe you could detect the changes to the git repository from another program (a shell script, a python program, etc.), then when a git change was detected, launch the build yourself.
For example, I have a job whose job name is "parallel". I can start multiple copies of that job by running the command:
If you were to probe your source master "every so often", then run that command when an actual source change was detected by your probing program, I think you would get the result you want.
That is just a work around, not a real fix, but it would let you experiment further with the idea of concurrent builds, without waiting for a fix from the Jenkins developers.