I honestly have no idea, I know the Restricted C96 values since I have one, and you are right that they getting expensive. As a 12.6 gun I would think the value would diminish to 50% of Restricted value, apples to apples condition examples.

In the interest of preserving these guns from the smelter, a professional rebarrel to Restricted length would be a good option, however I cannot think of a smith that could/would tackle the work. I know some Lugers have been successfully converted and it does boost their Canadianicon market value as they are then available to younger, non-12.6 licence holders like me.