seven π loudspeaker frequency response

Measurements below 200Hz in-room show the room's response more than the speaker's. So there is no real way to do it. One thing that could be done is to find a stadium that is open and/or very large and that has a corner to place the speaker in. Then you can load the speaker with a trihedral corner without enclosing the speaker in an indoor room. That would work. Or you can measure the speaker outdoors in the open and add 12dB/octave low-pass rise to the curve, simulating room gain. This gives a good estimate, but most rooms are more lossy than that. That's both good and bad, because the same things that prevent a full 12dB/octave bass boost are what damp room modes. Still, you can expect several decibels rise below 100Hz, making an overdamped curve like shown below what you want for corner loading:

Response of seven π bass bin without boundary reinforcement
Add 12dB/octave bass boost from corner loading