Trying to not have to make multiple pastry creams if I can just add to a base pastry cream.
Yes, to the zest, though I would not add lemon juice. It will affect the consistency of the pastry cream and I'm not sure that the acid in the juice would not affect the stability as well.
I think you can add the lemon zest or a few drops of lemon oil (food grade) or lemon extract.
I have added both lemon zest and juice to pastry cream - but I added a small amount of lemon juice. I used the juice from 1/4 of a lemon for a batch that made about 2-3c of pastry cream. It definitely brightened the flavor.
I think lemon oil sounds good though, I'll have to try that next time!
I added it after it was solidified so it had set up properly by the time I added lemon.