Not a bad idea - I've seen some down-firing passive subs that do this. There would be some extra cost (building a sub with a separate panel for these connections), but I think the biggest challenge would be how much extra height would be required. Unless you restricted users to only using 90° connectors for the audio and power, you'd need a significant gap under the sub for the cables to turn without crimping. My sub is at least a couple inches off the wall right now, and can't get any closer due to those bends.