Sealing a water pipe leak with hardening compound without disassembly
A temporary fix could be achieved by using a gauze / cloth tape, plus something like Fernox LS-X.



If you apply the Fernox to the joint, and then wrap it with the gauze, ensuring that the gauze is fully impregnated with the goo, you should get a good seal. Ensure you remove the pressure from the supply first.

Fernox actually uses the water to cure, so it can be applied wet. It's magic stuff - I always keep a tube handy for emergencies.
(I think this suggestion is probably remembered straight of the back of the box, btw)