This is an excellent suggestion. It's good enough that I used the company time machine to port it back to our iteration planning two weeks ago. Sadly our UI engineer is picky and he hasn't been satisfied with any of the five prototypes he wrote during last week.
So, that makes it two of us who are eagerly waiting for the ability to rotate work planes.. :-)
This is now live, you can activate it using the workplane tool and selecting "Adjust". The UI will change a bit, give us feedback through the app if you have input.
Have a dial and text box for saying the reference plane should be "45 degrees to the Z axis"
This would allow one to use the existing tools a bit more precisely.