Perhaps I should add that I've been trying to imagine whether movable "walls" (light weight but acoustically reflective barriers) might be used to block room reflections from reaching the mic. The movable walls would create their own reflections, of course, but if they are sufficiently clean--meaning not corrupted by a thousand reflections from whatever junk happens to be cluttering the room--the reflections might be significantly reduced via autocorrelation. Plus moving the walls would change the arrival time of their reflections at the mic, and help distinguish the reflections from the direct sound. If the approach worked perfectly, the results would be the same independent of wall-mic distance, so there would be a way of determining over which frequency range the measurement results are to be believed. Barriers of practical size won't work down to 20 Hz, but maybe they could at least be another way to gain a few msec of clean impulse response.
I've tried very thick walls of absorbing material around the mic but never got very far with that approach. I've been left wondering whether an "if you can't beat them, join them" approach might be better. Admit there will be reflections, but make them sufficiently clean and identifiable so that they can be more readily removed.
I haven't tried this in practice yet, but I was hoping someone might be able to help me avoid repeating work others have already done.
Few