I would be concerned that sitting directly on the vent might cause some damage to the plastic around the vent from that high a wattage. You could always try it and just keep a close eye on it. Double check your lamp to make sure that it is rated for a 100 Watt bulb. Many of the lamps we use with the cages are only rated for 60-75 watts, and the fixtures will become brittle from constant high temperatures from heat lamps. The heat lamps with ceramic sockets are designed for high temperature bulbs, but the plastic sockets are usually rated lower. Also, you'll want to monitor the temperatures inside the cage carefully to make sure your animals don't get overheated.
Good luck.
Dianne