Update. Starting to make sense now. At least one of the lights has gone on
, courtesy of one of the forum members (whom I would credit here, but I can't access the e-mail account who contacted me on from work right now), and the inimitable Bernie Hutchins.
One of the critical differences between flangers and phasers is that, where the number of notches produced by a phaser remains fixed according to the number of stages, as a flanger sweeps "lower" (i.e., adds more delay) the number of notches increase, with the notches being very closely spaced in the lower part of the spectrum, and a tank-y or box-y or metallic-sounding toen resulting if one introduces any resonance whatsoever. To be fair, in more complex phaser designs like 16 and 24-stage phasers, many of the notches produced are largely inaudible until the unit starts to sweep lower such it
seems like there are more notches as it goes lower. But for your meat-and-potatoes 4-6-stage phaser and your meat-and-potatoes 1-12msec MN3007/3207-based flanger, that's how it works.
The "theta" idea attempts to essentially stretch out the spacing of the flanging notches in the lower realm by applying a
frequency-based phase-shift to counteract the notch-spacing produced by
time-based phase shift.
What made the lights go on for me was when I
finally 
noticed that the phase-shift stages shown in the Bernie Hutchins paper and the Storm-Tide schematic had the cap and resistor reversed.
Normally, in the MXR Phase-90 type pĥase-shift stage we are all accustomed to seeing, there is a cap between the previous stage and the non-inverting op-amp input, and a variable resistance (LDR, FET, or other) to ground. That little network forms a high-pass filter. As the resistance is varied, the 90 degrees of phase-shift attainable by that stage is applied either higher-up or over more of the spectrum. However, at all times, the
least phase shift is applied to the lowest parts of the spectrum. If you flip those two components around, such that the cap now goes to ground, they form a
lowpass filter, and the potential phase-shift from that stage is always maximal at the lowest frequencies and minimal at the highest ones. I always forget which is called which, but the terms used are "phase-lead" and "phase-lag".
Theta processing involves adding some fixed phase-shift to the
lower portions of the delayed signal prior to mixing with the dry signal. The net result of applying both frequency-based phase-delay (from the allpass stages) and time-based phase-delay (produced by the BBD) is that the notches are spaced farther apart at the low end when the flanger sweeps downward. In principal (and apparently in practice), this will sweeten the flange sound and reduce the irritating metallic quality at lowest sweep points. I am VERY slowly wading my way through the Hutchins and Haible material, but I think this is the gist of it.
What it suggests is that there may be "reduced" versions of theta processing that could be applied to DIY and commercial flangers, in the form of a small add-on board. That add-on board could conceivably be between the delay path and mixing stage, or it could conceivably be in the recirculation path only such that it really only kicks in as the regen is turned up. A "reduced" version might be as trivial as two stages of unity-gain fixed allpass between the regen knob and the return point on the main board. Use a SIP chip and vertically-mounted resistors for that, and it could be conveniently inserted ibn plenty of commercial pedals....assuming it does the trick.
At this point, I'll shut up and let the more mathematically inclined pitch in. But I think I just found something to incorporate into my ever-evolving PAiA Hyperflange!
