Is it safe/energy efficient to surround hydronic baseboard heaters with a wooden enclosure?
Metal is used because it is a LOT cheaper than a wood cover.

You can use wood covers, but you have to leave the top open to allow heat to escape (or have a grill). Also, you probably should make the opening at the top somewhat wider than with a metal cover. I would probably put a strip of metal on the inside of the baseboard to protect the wood from heat. The design is kind of like this:



If I did this I would also drill a lot of holes in the bottom of the outboard part of the heating frame or leave a good-size vent at the bottom somehow. You want to maximize the airflow. Cold air entering bottom, gets heated, exits through top grille.