Understand but why would you ever do that when you can just read the SWR normally?...
It depends on what you want to do. The SWR bargraph on my old TS-450 shows the
SWR independent of output power (as long as you're over 2 watts or so) and will
the reading is steady enough with modulation that I can adjust the tuner while
calling a station if I'm in a hurry to make contact. That's not my usual operating
style, but it could be done for someone who snag a rare station before a pileup
starts. I built a similar circuit so I could tune up at 5 watts and then switch to
100 watts without over driving the meter.
My point was that there are circuits available now (and have been since the TS-450
came out something like 20 years ago) that will read SWR on an SSB signal (as long
as you are modulating, of course.) If the operating in the OP was using such a
meter, then the symptom of the SWR increasing with modulation type would be
due to a different cause than if they were using a manually set meter and just
had a higher peak output power. I'd agree the latter seems like a more likely
cause, but there are other possibilities. Besides, with a 100W rig if the SWR
were originally measured with 100W of CW, the voice modulation shouldn't
drive the meter up as far without being limited by the ALC.