Never thought of smoking with the roots, but I don't see why it wouldn't be safe if it's dried, especially if harvested during normal growing season while more of the sap would be in the trunk and limbs...although, you're talking maple...more sap should be even better!!! Flavor and aroma should be similar to the rest of the tree.
Just so you know (you probably already do, but others may not), tree roots tend to force their way under, into, around objects in/on the ground, and can cause unpleasant things to happen to structures. These roots you cut, or their siblings, may grow into this area you made and mess with your project at some point in time down the road.
Eric