I fannied about with this in the UK when I was trying to make lager. Kaffir Lime leaves. Lemon Grass. Lemon/Lime zest. They'll all give you a citrus, but it'll be knocked into touch if you over hop, or go heavy on the malts - i mean brew darker beers). Use about 5 Kaffir Leaves to 22-25 litres (more if you're more hoppy than a Corona), boil the leaves in some water and cool, dumping into the secondary/late ferment. If you go lemon zest (the juice & pith will just add sour and bitter - you might like it), I reckon about 2 to 4 fruit for 25 litres. Lemon grass is more subtle, but I've not used it.
What I settled on was a whole jar of Roses Lime Marmalade dropped in just before flame out (just the contents - not the jar!! - you could add it to your dissolving malts before you boil your hops), two limes worth of zest when I pitched the yeast and two limes worth of zest into the secondary.
FWIW, i used 1056 yeast (which I believe is pretty much US05).
I made it for a mate... he drank it... he said it was OK? Personally, I'm not a lager drinker - but fun making it.