Don't use a compression driver midrange. One of the design features of the π cornerhorn is overlap between bass and midrange that mitigates floor bounce. Another is midrange output low enough in frequency that most of the fundamentals are covered by the midhorn. I'm not opposed to using a midwoofer at midrange frequencies, but I don't like splitting the band there. Middle C is 260Hz, 400Hz is around G above that. I prefer to keep the crossovers outside the two octave band from middle C up.

Bottom line is, don't do it. You can make a speaker sort of like a π cornerhorn with a 400Hz crossover, but it won't sound anything like the ones we make. I'm not trying to impeach your idea, but I have made speakers like that and I found the mids detached from the bass. I prefer a two-way setup with crossover outside the C4-to-C6 band to a three-way with crossover in that frequency range.