Composite sync goes 300mV below “black” level (which is 0V if the white level is +0.7V) and above +0.7V (chroma waveforms) so the clipping you need is slightly more involved.
You could very likely use one of my DIY composite-to-eurorack bidirectional converters (1IF InterFace – Visible Signals) plus a ‘Wrangler’ video voltage processor module (Wrangler – Visible Signals) to achieve pretty-much your desired outcome (the Wrangler has an 0-1V clipped output). To retain the sync portion of the signal through the clipper output use the Wrangler voltage offset control to increase the output by +0.3V (most display devices will work perfectly fine with a small DC offset). You’ll probably still lose a bit of chroma on bright portions of the image though.
But then again, what encoder are you using? Most encoders will already output very stable and in-spec composite signals. The CBV001 is capable of quite some strong glitches, though - your best bet there is to use an external mixer with a tolerant TBC built-in (like the venerable WJ-AVE5 etc etc).