There may be a more uniform way to do all this that I'm not aware of. root.tcl.call() and root.tk.call() would be most symmetric while not creating extraneous windows, but that's not an option for the former, thus the methods I ended up using look different for Tk and Tcl.

Whatever the output, a tuple (8,5,9) or string "8.5.9" (easily converted to a tuple) will be nice, hiding any disparate implementation details.