In TclZlibInit (tclZlib.c) the package config for zlib is built and registered:
cfg.key = "zlibVersion";
cfg.value = zlibVersion();
cfg.key = NULL;
Tcl_RegisterConfig(interp, "zlib", cfg, "ascii");
The encoding should be set to 'iso8859-1' rather than 'ascii'. 'iso8859-1' is a built-in encoding whereas 'ascii' must be loaded. The two encodings are identical for characters in the range \x20 - \x7E (see http://en.wikipedia.org/wiki/ISO/IEC_8859-1\), which covers this use case.