I'd imagine it would be somewhere beyond where your toilet is typically clogged. That would handle most overflow situations.

A more practical approach might be a way to stop water from going into the bowl. E.g., you lift up on the handle to close the tank flap. Or maybe there's a stop button next to the flush button.

That would be almost as good as an overflow drain, but wouldn't require adding second sewer pipe.

A solid linkage between the handle and flap is available.

(The toilet in the room a little ways from where I'm sitting has one)

Behold!

http://postimg.org/image/ecyg84yof/

Overflow completely overrides the effect of S-bend. There is a (pretty important) reason behind S-bends.

Simply add an additional S/U bend to the overflow. Less likely to get clogged as the overflow is hopefully rarely used.

The overflow is only for water, right? Then a filter at the overflow entrance (or just the opening being narrow enough, two/three small holes maybe) could make positively sure the overflow bend cannot possibly get clogged.

my bathtub is like 1" lower than the rim of my toilet

oh, crap.