The Reface DX can actually do a fairly accurate resynthesis of analog style pulse width modulation -- taking a square wave to a narrow pulse and back - under the control of a LFO or EG. This video shows an example.
Also, get the patch here on Soundmondo: https://soundmondo.yamahasynth.com/vo...

A few things -- the basic square wave that the Reface produces isn't very "square". Truth be told, neither are alot of them on many classixc analog synths. The key is that the 50% pulse wave, aka "square" has only odd harmonics present, and that subsquent pulse waves created have the nth order harmonics missing for the duty cyle of 1/n. Meaning a 1/4 width pulse wave is missing all the 4th order hamonics (4, 8, 12, etc).

On Reface DX I'm relying on a back door to Operator phase control, as shown in the video. To 100% create controllable PWM you need to modulate a square wave carrier with a sawtooth wave modulator what is 90 degrees out of phase from the carrier. The modulation index then controls the pulse width You can do exactly this on the SY99/77 as shown here:
   • SY99 PWM - The Real Deal !  True Puls...  

Similarly, the FS1r and Montage are capable of doing this as well. The "All" spectral waves (the sawtooth wave spectrum) can be used to modulate the "Odd" waves as the process of 'Skirt' to increase the harmonics creates phase shifts in the wave that are close to the 90 degree sweet spot to create the PWM effect. Plus each Operator has it's own Pitch EG to refine with the pitch blip trick.
Sonic example on Montage here:
   • Tech Talk: MODX/MONTAGE FM-X