I haven’t tried it, but it’s possible that TBC2 partly recognizes it as a valid format, even if there is negative sync only. However, it happened to me a couple time to have the Composite output of VU007B connected to Analog to SDI + Video Assist while having the sync gen set to an HD format, so it would only output the negative sync, and the format would briefly show up on the screen, though not stable because of the missing positive sync.
Lars posted interesting circuits around the LMH1980 here DIY Circuit Snippets with the LMH1980: Frame Sync Extractor, Trisync/Csync Regenerator, Interlaced Detector, Activity Detector
including a trisync regenerator 
It has a couple additional features, like output enable and it also turns off the positive sync when in SD mode, features that you might not need if you’re generating your sync digitally instead of extracting it from an external source.
So it is basically summing the negative sync (labelled csync) and the positive sync (labelled hsync) using an op-amp and voltage dividers to bring them to the desired level. Since you’re dealing with +3.3V and the schematic is +5V, you’ll need to adapt both divider to reach the right level.
Also note that since the resulting sync output is 75R output impedance, and the receiving device will have 75R input impedance, you need +/-600mV sync amplitude at the output of the op-amp, which will then be divided by two when the transmitting device is connected to the receiving device (resulting in +/-300mV).
In VU007B, I’ve used +/- 600mV reference voltages and analog switches, with the switches controlled by the sync signals, though seems like the op-amp summing/scaling is more straightforward.