Rust doesn't keep any user from building Rust for Tier 2 or Tier 3 platforms. There is no separate configure guard. All platforms that Rust can build for, are always enabled by default. No one in Rust keeps anyone from cross-compiling code for sparc64 or powerpcspe, for example.
So if you want to copy Rust's mechanism, you should just leave it as is and not claim that users are being confused because "m68k" shows up in configure.ac.