That looks similar to Steve_B's design. If you haven't seen it check out
www.burton-manor.co.uk/Audio/HCLA.htm
, and read the whole site while you are there.
Renkus-Heinz also used a similar co-entrant mid/hf design in one of their products, I can't remember which.
The mid drivers need to be suitable for a horn ie light cone, strong motor, high resonant frequency. Ciare 6.38mr3 or 18sound 6nd410 spring to mind here.
The first plan appears to use two (10 inch?) drivers in bandpass enclosures for the low frequency, whilst the second is a sealed box.
