...Would it be possible to add a fire toggle for Wild Chunks?

This isn't currently possible because we have the Minecraft gamerule "doFireTick" set to be off and as a result fire will never spread or go out on its own in any of our worlds. It would be possible to change this over to our own fire prevention system however this would involve removing the ability for fire to never extinguish when placed on any block (a feature many players have come to like and use), plus we might reach issues with fire effecting blocks on the edges of your land. If enough people were to want this is might be something we'd consider looking into and possibly implementing but in our current set-up it isn't really possible.

One way you could remove your trees currently would be to use the mcMMO woodcutting abilites like treefeller or to get people to help you with their removal.

I do use Treefeller, but it doesn't work on those 40 block high jungle trees with a 4 block trunk. 

The other problem is that the trunks of the offending trees are outside my property lines and the canopy is 50 meters up in the air. I do use the Jump capability of the Clock in game, but still, take out the wrong wood block and you find yourself on top of a rapidly disintegrating bunch of leaves.