Cel, the Hsus have the controlled directivity benefits of horn-loading, so yes, they are horn-loaded speakers, they just don't have the hotter treble as most horn-loaded speaker because they aren't using compression drivers. Due to throat which compression drivers use to emit sound, they pretty much have to use an acoustic guide like a horn, but a horn can be used to direct any kind of sound producer, even a large woofer. Check out the old Altec Lansing "Voice of the Theater" which, once upon a time, were the most widely used speakers in commercial cinemas:
Also, rear porting doesn't really suffer unless the port is seriously blocked. Remember that the port is producing the lowest frequencies of the speaker's tuning point, so as long as their is some space around them, it ought to sound fine due to the lower directionality of bass. You just don't want to totally block or enclose the port area, or else you lose the port's resonance, same as with a ported subwoofer.
In the OP's case, I think the controlled directivity of the Hsu's horn will be less adversely affected by a cabinet placement then the NHT, but it isn't an ideal location for either speaker. The OP may also want to look at Klipsch RB-51, 61, and 81, those are front-ported, horn-loaded speakers which use actual compression drivers. The OP will definitely want to use room correction equalization as any speaker's response is going to affected by that kind of placement. The Classic Threes look awfully nice, but stereophile measured sensitivity at 83 dB, so they wouldn't be a good speaker for anyone who likes their music and movies on the louder side.