Yes, the more I look at it, it's clear that the claim to support other platforms was premature. Racket CS can now work in principle, but some pieces need to be filled in.
For a start, the Chez Scheme configure script shouldn't require a machine to go with --pb. You can get past that obstacle by providing any machine type with -m=, even using a made-up machine name. A good generic choice might be a pbarch name like tpb64le. Still, there's no longer a reason to require a machine type.
For the corners where pb mode needs machine-specific configuration, the intent is that "version.h" detects the machine as needed. For example,"version.h" recognizes ppc64 to enable big-endian mode in the runtime system... but that's not right for ppc64le. No doubt other things need to be fixed or added in "version.h".
Meanwhile, Racket's configure script looks for specific architectures to pick a pbarch machine type, and it also still requires a non-pb machine type. We can fill in more cases, but there should also be a way to specify a pbarch variant directly.
Probably the way forward is to fix configure scripts to not require a non-pb machine type, and then see what happens when building for different platforms.