I want to make a tempered white chocolate coating with brandy flavour. Will adding the alcohol ruin the tempering process, or is there another way of going about this.
There are two issues, both of which make this a bad idea:
You temper chocolate to give it a hard, snappy texture. Adding any liquid would be counter-productive to this goal.
All normal alcohol has some percentage of water in it (as much as 60%, for example, for a typical vodka). Small quantities of water will make chocolate seize turning into a grainy, paste-like mess.
If you really want to flavor chocolate that is to be tempered, you should use a pure essential oil. These have no water, and are used in very small quantities.
Still, the better bet is to flavor the filling or other ingredients in whatever you are making.
Saj already mentioned seizing. To expand: chocolate is a mix of starch and fat. If you add any liquid capable of hydrating the starch, you have to add enough to hydrate all the starch,. Else the starch clumps. The minimum amount of water you can add is 27% for milk chocolate and 39% for dark chocolate. I couldn't find numbers for white chocolate, it is very special as it only has cocoa fat and the remainder as fillers. If you add that much liquid, you end up with a thick ganache, not temperable chocolate.
I don't know what you want to cost. If you want to make chocolates, the standard is to put the alcohol in the filling . Even if it is a chocolate based filing, it can have liquid, as these filling s tend to be soft anyway. Of it is a cake, it is standard to coat in ganache not tempered chocolate. In both cases, you can add brandy.