This is a common request in one form or another. A common variant is that people would like to have numeric constants #defined rather than folded into the code as literals.
I wish I had a better answer for you today, but the Custom C Code solution is what I would probably do. I might write a build script that generated a header "on the fly" based on the current setting to keep things from getting out of sync. The build-script can also save the current value to a .mat file, which can then be loaded from withing the function(s) being compiled using coder.load.
