My quick solution to this bit of detail is, of course, to repurpose the 2d6 reaction roll to determine how well a district or connecting tunnel has fared. The results are:
- Good (9+): Most if not all of a district or tunnel is in use. There are almost certainly intelligent inhabitants who keep things in reasonable condition; there are signs of wear and tear, but anything broken in a previous cataclysm or stage of abandonment has been repaired (unless the current inhabitants don't understand the original use.) On a 12+, the area is patrolled and possibly well-lit, if the inhabitants need light. Doors will not be stuck, but may be locked or barred. Supplies will be in good condition.
- Average (6-8): District or tunnel is intact, but has been either abandoned by intelligent inhabitants or is in the process of being reclaimed. Accidental pit traps and deadfalls caused by disrepair may exist. Some doors may be stuck; others may be broken. Any light present will either be accidental (phosphorescent fungi or left-over magic, for example) or temporary (brought by invaders.) Areas devoted to specific crafts or other uses may still have usable tools and materials (5+ on d6.) Not everything will make sense. Inhabitants have almost certainly moved in after the area fell into disrepair; intelligent inhabitants are part of an expedition or relocation effort; they may have installed traps aroun their camp.
- Bad (3-5): Section is in ruins. Tunnels and rooms will be filled with debris, but will still be passable in general. Any but the most durable supplies or features will be unusable except maybe as raw materials (wood splinters as tinder, for example.) Inhabitants will mostly be dumb beasts; any intelligent creatures are in transit or part of a raiding party. Traps will all be of the accidental variety. Any wooden doors are rotting and should have a chance of parasites.
- Very Bad (2): Collapsed tunnel (must be cleared) or district isolated by partial cave-in. Individual rooms are collapsed on a 5+ (d6). Just about anything else could collapse, depending on a party's actions. There may be tiny passages for small vermin to enter/exit, but nothing bigger will be alive in this section. Any major inhabitants will be enchanted, undead, or otherwise extremely dangerous, since it has survived for centuries without food or water.
I recommend rolling separately for each district and each connecting tunnel. This doesn't have to all be done at once; as I've mentioned, I'm seeing this mainly as an impromptu dungeon planning technique, so you may have one small section of an underworld mapped out and some collapsed tunnels to other, vaguer sections, which you would roll for when needed.