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The second N.T.S.C. (the one in the early 50's that set the U.S. color system) did experiment with phase alternation, but at field rate. They wanted to try this to allow both the I and Q signals to have asymmetrical sidebands, which would cause phase distortion at the edges of objects. There were no 1-line delay lines, let alone 1-field, so those sets had to rely on eyeball averaging of the phase. (The first practical consumer-priced 1-H delay lines were the galss ultrasonic lines developoed for PAL TVs in the 60's.) They concluded that there was noticeable flicker, and the phase distortion problem was eliminated by the use of extended lower I sideband only, with symmentrical Q sidebands. They did not directly contemplate using phase alternation as a means of reducing or eliminating hue adjustment errors.
By the way I disagree that SECAM is superior to any other color system - it requires all sorts of compromises in the system design to prevent objectionable cross-luma artifacts, and is impossible to use in even simple mixing systems due to the impossibility of linearly adding the FM subcarriers from two pictures. Production for SECAM countries is typically done in PAL, with a final conversion to SECAM for broadcast. If they had kept the concept of alternatng R-Y and B-Y lines, but used AM carriers instead of FM, it would have achieved the goal of hue stabililty without all the detriments that it has. Also, because of the frequency modulation, usign a comb filter to improve luma resolution is impossible in SECAM. (Color saturation is another matter - both NTSC and PAL can suffer color saturation errors if the color subcarrier is attenuated or amplified with respect to the luma, as in antennas or ghosted channels that do not have a flat frequency response, while SECAM will maintain correct saturation regardless.).
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