If you were to pursue this, then first determine the amount of power used with the IR on. Then measure the power when the IR is turned down to a level that will still produce accurate triggering but perhaps a not so good image.
Then see if the power saving difference is great enough to consider your original theory.
There are Dahua API's that can set IR LED levels. The issue is will they work with your model of camera or how can the API be triggered with your cam. The next problem is how long will the cam take to adjust to the new light level or focus needs (as pointed out by Flintstone61) in relation to when the image is taken. If the cam has alarm outputs then a trigger could turn on an external IR lamp. But again, the same above stated issues will apply.
The simplest solution is more solar plus more battery. But your statement "don't want to make any changes to this system at present" put you between a rock and a hard place.