The weather was nice enough this last weekend that I moved the maple syrup operation outdoors.
The trick here is that if you leave the lid off the pot, the ashes from the woodfire occasionally blow in. But if you don't leave it off, the steam doesn't escape, and it'll never boil down.
I voted for removing the lid, and having the Boy stand next to the pot and occasionally blow the ashes away. It wasn't effective in any way. But he enjoyed the opportunity to contribute.
(Actually, one of the last steps before the final boil is to filter the condensed liquid - so the product & taste'll end up just fine.)