You can easily render an empty, black MPEG-2 file the same length as the audio file. This would be the most straightforward thing to do. When authoring the DVD you can set the end action on the clip to loop which will play it continuously. There will be a slight gap before playback starts again. The length will be dependent on the player and how quickly it can seek the beginning again. It may be short, but you can't expect gapless looping.

If the video ends up being smaller than 1GB then you can use the video/audio as the background media of the main menu itself and set the menu to loop. This way the staff wouldn't even have to press the play button at all. Simply turning the power on would start it playing. Making the video file small enough should be easy if you aren't including any video that needs to be seen; simply use a low bitrate for the video render. 20 minutes of 16 bit 48KHz stereo would be less than 250 MB. 20 minutes of video at 1mbps should be around 150 MB. Combined that's 400 MB which is well under the 1GB limit for menu media.

Another option is to create a picture compilation in DVDA using the wav file as the background audio. One single picture that lasts the entire length of the audio would work. I've never done one of these but i presume you can set an end action there as well. However, making an actual video is more likely to be compatible with a wider range of players.