Already implemented in LabVIEW. Available in LabVIEW 2010.
I feel that it would be very handy to be able to define your own math constants, and mask them similar to the ones already in labview, much the same way one is allowed to design the icon for vis.
In my specific case, I do a lot of heat transfer and flow modeling, making pi/4 a much needed constant (pi/4*Diameter^2). It is clunky to have that simple division all over my vi's and masking as a subvi seems over kill.