Reversing glyph sequence

I'd like to be able to do things like:

sub A A B B by B B A A;
sub A B C D E F by F E D C B A;

I don't see how this could be achieved reading the spec, and FontTools errors out with an "Direct substitution of multiple glyphs by multiple glyphs is not supported" if I try the above, but I was wondering if there's a trick to still accomplish this.