How Can a Pinhole Leak be Temporarily Fixed Until a More Permanent Repair Can be Done?

Ordinary 2-part epoxy resin also works well for sealing leaks. I have used it successfully for repairing a long crack in the polystyrene spray nozzle of a hose. Make sure you get the quick setting stuff which hardens in about 10 minutes. Try and dry the joint thoroughly, and if possible, rub it with sandpaper to improve adhesion. Let the epoxy stiffen slightly before applying to prevent it spreading out. Heat from a hair drier accelerates the curing process. As far as I know special pipe sealants are epoxy based.
Another alternative (although this is theoretical and I haven't tried it!) is to use a hose clip. Keep one opened out and if you have a leak, wrap it around the pipe, push the end of the clip back into itself and semi-tighten it with a screwdriver. Push a small piece of rubber (bicycle patch, piece of old tire, rubber boot or whatever) under the clip over the hole and tighten. The advantage of this is that the water wouldn't have to be turned off and the area around the hole wouldn't need to be dry but it would probably only work on a hole in the actual pipe itself where the clip would exert sufficient pressure on the patch. You can also buy emergency clamp type fittings which seal over a hole, effecting a temporary repair until the section of pipe can be replaced.
Yet another repair method for cracked brass fittings is to solder them. I had to do this when a fitting on a heating system in my workshop developed a hairline crack during a lengthy period of snow during the winter. (Haven't looked at it for years so hopefully it's still water tight!).
It's wise to know the location of all your stop cocks/gate valves, and write the function of the valve on the wall behind it with a marker, or tie a label around it. In an emergency, you don't want to have to figure out which valve shuts off which pipe. Also valves tend to stick and seize up after years of disuse, so "exercise" them at least once a year by screwing off and back on again. It's actually a good a wise idea to add a redundant valve in case your main water shut-off valve fails. Another option is to fit a quadrant ball valve in addition to the main valve which can be quickly turned off by turning it through 90 degrees (These are the ones commonly encountered on gas and air compressor lines or on oil tanks). Quadrant valves don't normally seize up.