Interesting comments from Linkwitz Labs. They pretty much eschew the idea of rear facing tweeters or indeed dipole tweeters
Q25 - Why do you not use a rear firing tweeter or a dipole ribbon tweeter?
A25 - A rear firing tweeter for the PHOENIX would increase the speaker's high frequency 4p acoustic output by 3 dB for an even more uniform power response. In earlier designs I actually used such a tweeter. Because of the physical size and separation of the two tweeters - in terms of wavelength - it is impossible to obtain a simple cos(a) polar response. Instead, the off-axis response will consist of multiple beams, where the outputs of the two drivers see each other, which is primarily off-axis. On-axis, in front, the rear tweeter contributes very little to the sound. Overall I have not found a real benefit resulting from a rear tweeter. The off-axis response of the single tweeter-midrange combination is well behaved and actually has its widest dispersion around 4 kHz. Above that point the normal increase in tweeter directivity comes into play. Also, a rear tweeter may cause reflections from nearby objects which affects imaging and makes speaker placement critical. I am therefore using only a front tweeter.
A dipole ribbon tweeter would have the same potential problems as a rear tweeter. In addition the low 1440 Hz crossover requires large volume displacement capability. Typical high frequency ribbons would generate severe non-linear distortion under this condition. A ribbon of greater length would narrow the vertical polar response too much. The large amount of air movement due to the close proximity of the 8" drivers might modulate the movement of the low mass ribbon and lead to further distortion. I have not used a ribbon driver, because the ones that I am aware of do not seem to fit this application.