I am making crème brulee for 60, I intend to sous vide them (thinking 180 degrees F, roughly 83C?), and I would like to create an "old fashioned" flavored crème brulee. To accomplish this, I intend to add brandy and bitters to the base. Does anyone know if there is a point where too much alcohol could cause problems with brulee? Any advice for appropriate amounts or ratios of alcohol to base?
The alcohol does indeed change the coagulation of the egg yolks, so you have to be wary. Small amounts are not problematic (I have made orange crème brûlée with a tablespoon or two of orange liqueur to a full batch), but if you want more for taste reasons, you should look closer at the recipe. I tried looking this up, but couldn't find an upper limit on the alcohol amount - although note that eggnog can be made without curdling, so there is quite some tolerance in eggs.
For the maximum alcohol that is usable, I would look at sabayon recipes. Pick a recipe that is meant to make a firm enough version, and look at the total amount of pure ethanol. Since marsala has less ABV than brandy, you will be able to add more of the brandy - for practical purposes, you may want to fill up the cocktail with enough milk (or cream) until you are around 17% alcohol, then use this as a substitute in the sabayon recipe. There is a very good chance that this goes well - while the fats in the cream will interfere a bit with setting, the mixture should have less acidity, which will again give you a good overall mixture. It is not predictable how further components in the bitters may interfere, you will have to watch the first few batches and adjust the temperature or ratios if needed.
Once that version turns out well, you will have established a good upper range of alcohol for your crème brûlée, and from there, you can adjust downwards for the taste you like.