A mushroom plug is an ideal fix. It depends where the hole is whether they will do them. In the sidewall it is not a good idea, but as you dont normally get punctures there and it is normally in the centre of the tyre on the contact patch. I have used them!
The frightening bit is when they reamer the hole out with like a big bradawl and grind the inside of the tyre to clean the surface.
Here is a link to best plugs and a little diagram to say where it is safe to use them!
http://www.rema-tiptop.de/automotive.../minicombi.php
You have to be aware that if water has got into the carcass, it can rust the steel belting and cause seperation. The object that punctured a tyre in the first place can also damage the carcass. Rare, but that is the reason you are told not to repair tyres.