I want to author a DVD with 2 Dolby Digital audio tracks. One is 1.0 mono, the other is 2.0 surround. I want to be able to switch between the two without sudden jumps in loudness.

Now I understand the basics of setting dialnorm so that dialogue is decoded at approximately equal levels. However, I don't know if the music/effects will sound quieter on the mono track because they are being represented by one channel instead of 2. In this case, should I ignore the dialogue level measurement and set dialnorm some other way? Is there any guidance on this anywhere, or I should I just encode test discs and set the relative levels by trial and error?

Stereo will only be louder than mono if you will be playing the stereo with two speakers but the mono with only one. I assume your mono will be played back through both speakers in which case they will be the same level so just use normal dialnorm.