winset(receiver, "*.some_macro_id", "command = some_command")
This snippet would alter any macro entries with the ID "some_macro_id" in any macro list to have the command "some_command". (So if I defined two macro lists in a skin file, macros_def and macros_alt, both lists, containing a macro entry with the ID "some_macro_id", would be modified with a single winset.)
The syntax seems questionable though. We can play around with it if this is considered.