I posted a quick sample file showing how a formula property can be used to have your room name underlined in a Schedule Tag but not underlined in a Schedule Table in a post in this thread in the ADT 2006 Discussion Group.
I did this using mostly out-of-the-box content from ADT 2004, making a few modifications to achieve the desired result. The RoomObjects:Name property was renamed RoomObjects:NameSchedule. This is a manual property and is where the user enters the room name, without a "%%u" prefix. This property can be referenced in a Schedule Table where no underlining is desired. A new formula property, RoomObjects:NameTag, was created, and the formula concatenates "%%u" with the RoomObjects:NameSchedule value. This provides the underlined version of the room name, for use in a Schedule Tag.
The Aec3_Room_Tag_P view block of the Aec3_Room_Tag room tag was modified to change the attribute definition's tag to reference the RoomObjects:NameTag property. This gets the room name underlined in the tag.
12/04/2015 UPDATE: Refer to this post for the revised method for doing this in later releases where %%u does not generate an underline.