You mentioned building dipoles, and the large Alphas - the 12 and 15 - have such high Q that they do pretty well on open baffles. Being a dipole, the bass will have dual lobe directionality and then up high, the circular radiator will start to have collapsing DI so you can match directivity with the horn. With the right baffle size, you could probably do a pretty good job of making DI fairly uniform throughout the band this way, with of course the exception of the rear dipole lobe.

For additional specs, contact Tom James, Jerry McNutt, or Scott Diehl over at Eminence. Being a circular radiator, the polars are fairly predictable. As for nonlinear specs, not many manufacturers are publishing nonlinear parameters yet, but Eminence has a Klippel system, so they can probably provide a dataset of nonlinear parameters for you.