It's been a long time since I actually did this, but my recollection is that the result will leave some residual water. It's not an ideal environment for a flambé, but that's OK because you're not actually trying to cook with it, it's more of a show technique that generates just enough heat to produce a caramel-like texture from ground-up caramelized sugar.
Some pointers that might help you here:
- Don't overdo it with the alcohol. You want to use a very small amount - just a splash.
- Make sure the alcohol is hot enough before you try to ignite it. Make sure that it is a uniform temperature, and that you're not just igniting the surface.
- Work quickly. If too much of the alcohol burns up while you're working, then you are essentially dumping water in, and there might not be enough heat to melt the sugar powder.
- Consider serving the crème brûlée closer to room temperature, or at least not as cold as fridge temperature. The colder it is, the quicker the fire will get doused and the more liquid will (probably) remain behind.
- Custards are firmer than pudding to begin with, so "watering it down" shouldn't be a huge concern.
- It's a bit of a cop-out, but I want to say, don't worry. If it's good alcohol then nobody's going to mind a bit of a film on the surface - it's part of the attraction!
I remember this taking me several tries to get right, and probably wouldn't use the technique at all if I were serving to a particularly critical audience. It's more about getting a passable crème brûlée with some cool visual effects than getting a perfect crème brûlée with an impossible technique.