Good substitutes for WD-40 in emergency situation?
Actually, WD40 is a poor penetrating oil -- Liquid Wrench, et al, are better.  But the Coke trick works, supposedly because of the phosphoric acid.  Another one is to heat the part -- eg, get a piece of iron hot in a fire and then hold it against the fastener for a minute or two.  Works best if you can actually heat the female part of the connection, but tends to work even if you must heat the male screw.  Sometimes heating and then dousing with water will break a part free, if heating alone doesn't do it.

Of course, be careful of any plastic parts, and avoid overheating bearings.