Are you referring to default settings similar to the one [provision][voicemail_number] = *97? Most of the existing templates match the default dialplan for DND.
You can theoretically put anything you wanted in to default or domain settings or device settings if it assists with provisioning.
Yeh, however I believe all the provisioning templates bundled with FusionPBX already hard code the DND number (instead of using a variable) to the dialplan's starcode for it so it's a bit moot.