Given that arc compiles to mzscheme, mz seems like the right name. If there was a CL-based arc with the same feature, the name would be more debatable. In rainbow for example, a "pass through the compiler unchanged" feature is meaningless - there is nothing on the other side that would understand.
"mz" also has the advantage of being short, expressive and euphonic, unlike "qac" or "ptac"
More generally, why would you want the pass-through-compiler feature to be named the same thing in different arc systems if the backend is different? Code that works when you're compiling to mzscheme is useless if you're compiling to CL; you'd rather have it warn you that it doesn't know what the hell you're talking about when it sees "mz" than have it (mis)interpret your mzscheme code as CL code.
