From my wandering around the ruins, the plant life looked the same as the rest of the area. My hunch is that the difference from the CO2 is smaller than if the plants were being actively fertilized, for example.
Compared to the damage done to the ground and surface water in that area due to mining, probably little-to-none. The fire is burning slowly, if indefinitely, and anthracite coal is relatively clean-burning.
I was thinking it would be beneficial to have an abundance of co2
Gotcha. Given how ecologically screwed up that area is, it seems like it'd be tough to separate that effect. We can at least hope for a silver lining.