I have just (yesterday) bottled 20L of a lemon hard cider.
I used the juice of a lemon (medium sized), and peeled the zest off making sure I left as much of the white as possible behind. The white stuff just adds more bitterness than lemony goodness.
On a side note, I don't think you would want to use a microplane or zester as you'll want to eventually remove these adjuncts, and large chunks means easier removing. If you'll eventually be looking to your hard cider, you might also want to filter that juice while you're at it to get rid of the pulp.
I added it all during the secondary, and for a total of a week. The lemon taste was peppy and refreshing, not overpowering. I would definitely not use much more than that.
The base recipe was old style cider + EC1118 + 2 cups of raisins.
I used xylithol at 15g/L to sweeten the hard cider before bottling with 10g/L of dextrose.
It kept me smiling while I bottled those 20L, plus 24L of melon&cascade dry-hopped hard cider
I'll let you know in 3 weeks how the carbonation went and if it'll fulfill its "Hot BBQ Day in July" mission.
