Maybe tweeter upward?
I don't think you would hear much above 8 kHz with the limited dispersion of most tweeters. You would be 90 degrees off axis. But it might be interesting to place a speaker with a concentrically mounted tweeter inside the woofer (I have a pair of those) facing up on short stands so that you weren't so far off axis. Hmmmmm. Maybe I'll try that.
