Most of your requests are dictated by the DSP.

24dB/ oct. and 12dB/oct lowpass crossover (selectable)

It only has a 4th order filter (24dB/octave - I think I said decade somewhere else) in mono sub/centre channel mode. In stereo sub mode, the crossover is 2nd order (12dB/octave)

Crossover frequency is not fixed as such but requires intricate knowledge of the DSP to select the 20 odd coefficients required. The manual supplies coefficients for 40,60,80,100,120 Hz only. These even change with sampling frequency which is not a problem on the empeg since it only uses 44.1kHz

gain

Easy and done.

180 degree phase switching via Hijack (more preferrably, fully variable or semi-variable phase adjust)

Technically very easy but the DSP code isn't programmable. Could be done in hardware quite easily though - wouldn't be hijack switchable then. I'm not sure about variable phase though. How does that work? I understand how you can do 180 degree shift by inverting the signal but not variable really.

it would be very nice to have a coule bands of fully parametric EQ

Again not without programming the DSP and that ain't going to happen.