I have to say that I have never run any but now I actualy was thinking to try and was curious to see what people think about it. Probably in the game there is not much You can do as 0 Lv character, but I think it can help determine background for characters until they find a trainer and level up to Lv1 and lets not forget cantrips and orisons.
I don't know much about it and I think it all started with Unearthed Arcana where Cantrips and Orisons were presented, as some rules for 0 Level Cavaliers and Wizards. Here is first link that I have found about it but those rules in GH Adventures are a bit more complicated then those in Adventure N4 Treasure Hunt http://www.therpgsite.com/showthread.php?15409-AD-amp-D-Zero-Level-Characters
Now second link that I have found is this which is mostly like it was explained in N4 Tresure Hunt https://forum.rpg.net/showthread.php?731518-AD-amp-D-Zero-Level-Characters-in-AD-amp-D&s=70ba932ecb365bf031323e1824636aee
Now for my campaign ( I would like to see something like this too in DC or FRUA, though I really don't know how it would go
I think it would be like N4 Treasure hunt rules (for now):
-500xp (or more depending of class)
1d6 HPs (min 3HP at start), reroll on Lv 1 for class and add or subtract till 3hp (min)
Roll 4d6s for stats (that is for me 8 times drop 2, this could be rerolled at Lv1 and took better but I guess that would be complicated to do in DC or FRUA)
One weapon only (that is permited by class character is going for, any that is allowed for that class)
Secondary skill roll only and no non weapon proficiencies at this time.