I think that FOR NOW, 'mz is fine. Even though each person has their own patched-up Arc, it seems as though the vast majority are based on pg's Arc on mzscheme. If somebody is using an Arc built on some other VM (eg Rainbow on the JVM), then they will be putting quite different code within the body of an 'mz, so I don't think it's an issue that they'd also have to change the name of the symbol.
