- Check for unused grounding wire, and connect if found (great option, but I suspect we will not find such wiring based on the age of the building)
This is the only option that will actually provide equipment grounding, assuming you find an equipment grounding conductor in the box.
- Replace receptacle with ungrounded GFCI outlet
While this will provide ground-fault protection; which is good, it will not provide equipment grounding. In fact, when you do this you have to add a label that reads "No Equipment Ground" on each receptacle protected by the GFCI.
- Connect a ground wire from the outlet plug to it's box (presuming we discover the box is metal not plastic) and use the box for ground
This only works if the metal box is connected to an equipment grounding conductor, otherwise it offers no grounding at all.
- Ignore the lack of proper ground and cross our fingers none of our electronics get damaged
This is probably the most common solution. Though most people aren't ignoring the lack of grounding, they simply don't know they need it.
- Run an extension cord from a properly grounded outlet in a non-original part of the house to where the electronics will be?
This is a temporary solution, but you'd have to protect the cords from damage and use the proper size cord.
There's really not an easy solution here, the only way to do this is to run an equipment grounding conductor.
Other useful questions:
Without being able to see the cables as they enter the cabinet; or the ability to touch or trace them, here is what I assume is going on.
Definitions:
Grounded (neutral) from the service
A typical single split phase service is made up of 3 wires. Two ungrounded (hot) conductors, and one grounded (neutral) conductor. The ungrounded (hot) conductors will connect to the main service panel through a disconnect (usually a large breaker), while the grounded (neutral) connects to the neutral lug. The neutral lug will be bonded (electrically connected) to the neutral bus bar, and all grounded (neutral) branch circuit conductors will terminate at the neutral bus.
Grounding Electrode Conductor
This conductor is used to connect the grounding electrode (ground rod, etc.), to the grounding bus in the panel. All equipment grounding conductors will be connected to this bus.
Bonding Jumper
The bonding jumper is used to bond (electrically connect), the un-energized metal parts of the panel to the grounding system.
Assumption:
Since it appears that (what I assume is) the grounding electrode conductor terminates at the neutral bus, I'm also assuming that this is the main service disconnect. This leads me to believe that the neutral and grounding buses are bonded (electrically connected). In which case, technically, grounded (neutral) branch circuit conductors can terminate at the grounding bus.
So you have two options:
Terminate the grounded (neutral) from the new circuit to the grounding bus.
Move the green wire that is terminated on the neutral bus, to the grounding bus. Then terminate the grounded (neutral) from the new circuit, to the freed up slot on the neutral bus.
Additional Information and Code Compliance:
Number of Conductors
Since this is a new circuit, it has to be installed to current code standards.
National Electrical Code 2011
ARTICLE 250 — GROUNDING AND BONDING
VI. Equipment Grounding and Equipment Grounding Conductors
250.140 Frames of Ranges and Clothes Dryers. Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.
Which in this case means installing a NEMA 14 receptacle for the dryer, and a proper grounding conductor.
You'll have to follow the dryer manufacturers installation instructions for upgrading to a 4 wire cord. For more information see this answer, and this answer.
Since you've said that you're already using 4 wire cable, you'll simply have to terminate the grounding conductor in the cable to the grounding bus in the service panel. Then connect the other end of the grounding conductor to the grounding terminal in the dryer receptacle.
Size of Conductors
You'll also want to be sure that you're using the proper size breaker and conductors. In the case of a dryer, you'll typically use a 30 ampere breaker and 10 AWG conductors (depending on the length of the run). However, you'll want to check the dryer manufacturers installation instructions to verify this.
Best Answer
Rewiring a house takes about a week, give or take, if the electricians have free reign. If they have to tiptoe around the occupants it could take a lot longer. The best way to find out is to have a few electricians do a walk-through and give you a quote.
The way I've seen upgrading a house's electrical service with minimal disruption is to add a new breaker panel next to the original fuse box and then attach the original fuse box as a sub panel. (E.g. if the original box is 60A, you would feed it from a 60A double-pole breaker in the new panel). The panel upgrade could probably be completed in a day (with the cooperation of your electric company, who will need to cut power to the house and maybe run new wires). Then any new circuits can be run off the new live panel. But I have no idea if that is up to code or would pass inspection. I suppose in theory you are supposed to get an inspection before any new or rewired circuit goes live, which might make moving the circuits one-by-one prohibitive.
You may decide it's easier to stay with friends/family/motel for a week instead of dragging the process out for a while. Also note that rewiring a house will put a lot of holes in the drywall, which is somewhat messy and will need to be patched / painted when it's all over.
P.S. - if you are renting, this is probably all speculative, right?