Hi:
Here is my take in this. First of all, I would suggest using a GS4881 instead of the LM1881. Unless NatSemi has changed the design of the LM1881, they have terrible temperature drifts. Mind you that this is from my experience of about 12 years ago!
Second, I do not think your circuit can work. The big reason is C2. In order to add Sync on Green, you need to clamp the Green signal to a known DC level, and the sync has to be below that clamped black level. Using C2 (AC soupling) destroys this DC reference.
Now, if, as I suspect, the Green signal from your scan converter comes from a DAC, then it should be ground referenced. In this case, you can try this circuit. It is an adaptation of a circuit I designed about 12 years ago to convert a standard VGA signal (RGBHV) to RGB SOG without using any external power. In most video cards, the RGB signals are ground referenced, and the syncs are TTL levels (positive or negative). Basically, the circuit uses the backporch signal from the GS4881 to generate a small negative voltage, which is then superimposed on the Green video.
Hope this helps,
Jem