I saw something like what your talking about before. The guy had two slot ports running under his seats and coming out at the front of them. (He was running 2 15" HX2's) that thing pounded! I guess you would build your box the size you wanted and you wouldn't have to worry about port displacement, so you could go with a smaller box. I'm not sure what size your A's want. I imagine ED's specs consider port displacement. If you could figure how big the box would be with out the port being inside of it, I'd say go with it. Or you could play it safe a talk to an ED tech.
2006 Jeep Liberty
Adire Audio, Cadence, Clarion, MB Quart
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)