Your description reminds me of something in between a malternative and the classic beach side invention of a cold brew garnished with a wedge of lime. I must confess I too like certain beers enhanced with citrus flavors. Lager and lime, wheat beer with lemon and Belgian-style ales flavored with orange peel are examples of such pairings.
Maintaining a fresh citrus flavor in beer can be very difficult — no matter if you attempt to get the flavor from fresh fruit added early in the process or from juice prior to bottling. This is analogous to maintaining a fresh hop aroma in beer. Most brewers agree that very few beers, even when dry hopped, maintain all of the hoppy goodness found in hops prior to their addition to wort or beer.
The common citrus oils (lime, lemon, orange and grapefruit) are all primarily comprised of a compound called d-limonene, a monoterpene that has the distinctive orange aroma. Bergamot oil (used to flavor Earl Grey tea) is not dominated by d-limonene, an oddity among citrus oils, and is primarily made up of linalool, an oxygenated monoterpene, and linalyl acetate giving Bergamot oil a floral character. The “d” preceding limonene is a stereochemical modifier and signifies that there is a mirror image cousin called l-limonene; l-limonene smells like fresh pine needles.
The compounds making lemon, lime and orange oil smell different from one other are a variety of other terpenes, such as farnasesne, caryo-phyllene and pinene, to name a few, as well as other aroma compounds. Interestingly, these same terpenes are found in hops and explain why citrusy and piney are two common hop aroma descriptors.
If you want to maintain the citrus oil aroma you should use the same practices employed to preserve hop aroma, such as late hopping and dry hopping. Belgian-style white ale or wit bier is flavored using the peel of the bitter Curaçao orange and most recipes I have seen call for adding this ingredient late in the boil. I have used Curaçao orange peel added in the kettle and to beer during aging and have been happy with both methods.
Peel oil is only one component of what one perceives as citrus flavor. The other key components are sugar and acid. Most citrus juices have a specific gravity of about 1.048 because they contain sugar, like other commonly eaten fruits. They are also high in citric acid. The combination of sweet and sour give balance to citrus juices and some are more drinkable than others. Lime and lemon juices are more acidic than orange and grapefruit and are made more palatable as a beverage by adding “ade” behind their respective name. To transform lime or lemon juice to limeade or lemonade involves diluting the juice with water, thus diluting the acid to a palatable level and then boosting the sugar level back up to the undiluted strength by adding sugar. Orange juice, grapefruit juice, limeade and lemonade all contain that “clean and crisp” flavor you are seeking.
When the balance of the juice is disrupted so is the clean and crisp flavor. This problem is not unique to citrus beers and many fruit beers turn out to be a disappointment because they lack the fruity flavor of the juice used in the formulation. One reason is that the fruit sugar is fermented and that component of the fruit juice flavor is lost. In the case of wine, that’s a good thing since the goal is not to produce alcoholic grape juice unless of course we’re thinking of Mad Dog 20/20! The other component that is diminished is the acidity and that is primarily caused by dilution.
Okay, now for some frank advice on what you are projecting. Adding acid to the finished product is a common method used by commercially produced citrus flavored beverages. Go to the store and read the labels on non-alcoholic citrus drinks and you find that many contain various acids, with citric and phosphoric being the most common. Lowering the pH of a drink gives it that acidic zip you seek. This method will help achieve your desired result.
Once upon a time I experimented with a blend of high-quality fruit juices, malic acid, lactic acid and acetic acid (found in lambics) to produce a blend that could be added to light beers (such as wheat beers) to produce a pseudo-lambic. The resulting beer was delicious, but the blend was too expensive to be commercially viable. We sold a pilot run of this elixir to a brew-pub and it cost $90 to flavor one keg — unfortunately far too much for most brewpubs to afford. The key to this brew was the acid blend. Without it the beer was too cloying and lacked the crispness and drinkability you desire in fruit beers.
You can add the acid in the form of lime or lemon juice if you simply add enough fruit. Although many brewers cringe when people want fruit in their beer the result is quite refreshing. I personally enjoy wheat beer with lemon and light lagers with lime. To really get that fruity tang, one quarter lemon per half liter gives a good, assertive flavor. This translates into the juice of 10 lemons or limes per 5 gallon batch. So, you could add this juice right before bottling, but the juice is fermentable and there is yeast in any unfiltered beer. This means you would have to filter and preferably pasteurize (or chemically stabilize your brew) to prevent fermentation. This is not an attractive alternative to most of us, who appreciate the flavor components of unfiltered yeast in a brew.
My honest and least sexy suggestion is to do what so many beer drinkers do when they want a lemon or lime flavor in their beer . . . cut a piece of fresh, juicy citrus fruit, hold it over your glass, give it a squeeze and drop it in your brew — simple and effective!