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It would be a fairly simple matter to make a 4.5 MHz "pickup" coil with no actual connection to the set and build an all-tube outboard 4.5 MHz SIF, Detector and MTS decoder based on a modified FM-MPX design and connect that via RCA jacks to my surround receiver.
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I used to be a TV research engineer at the old RCA Smirnoff Lab back when stereo TV sound was developed. One thing RCA found that they had to do was have a separate "video" IF amp that got the signal from a tap on the tuner, bypassing the actual video IF amp. The reason for this was that the true video IF strip had a sloping passband at the picture carrier. This was done because of the vestigal lower sideband carrying the video signal. "Nyquest slope" it was called. Anyway, this slope causes some residual incidential phase modulation mostly from the vertical and horizontal sync pulses to get imposed on the picture carrier and when that is beat against the sound carrier, adds some contamination to the demodulated FM sound. So RCA used a special "video" IF without the slope on the picture carrier to avoid this. This also kept the intercarrier sound system to ease TV station tuning.
So you'll need to build such, or more likely easier to do is use the video IF from a junked B&W TV set, and set the IF transformer slugs to peak at the picture carrier and another peak at the sound carrier. And use the existing 4.5 MHz sound IF amp and detector. At this point it should sound very clean, no buzz. Remove the FM deemhasis RC circuit and feed the audio with MPX signal to your stereo decoder.