The Tcl/Tk devs have closed the issue I opened on their issue tracker as "not a bug", with the following comment:

"Because Tk on the Mac uses API's to draw buttons, some configuration options are set by the OS and not by Tk. This is documented in the button man page: "On Aqua/Mac OS X, some configuration options are ignored for the purpose of drawing of the widget because they would otherwise conflict with platform guidelines. The configure and cget subcommands can still manipulate the values, but do not cause any variation to the look of the widget. The options affected notably include -background and -relief." Closing this ticket as not a bug."