So I read the 'prime pump method' from the comment above... Assuming there are no other problems with that technique, one issue I see is that the spray bottle distributes a specific amount of sugar every time at the same store-bought, pre-mixed concentration (whatever amount is in a given brand of simple sugar mix). This is good for consistency (all beers come out at exactly the same amount of carbonation), but bad for matching the appropriate level of carbonation to whatever beer style.
In other words, if the level of carbonation is an important part of beer, like hops or some other component, then it seems we may want to add the appropriate level of carbonation to a given beer, just as we might with hops.
For example, if I told you that the amount of hops you want to add to every style of beer is 4 oz in a 5 gallon batch, that might make you a pretty good beer every time. One size fits all. However, if we really wanted to be accurate about the style of beer we were making (Imperial IPA, Berliner Weisse, etc), you may need to add more or less hops to brew a good example of the style.
Accordingly, every style of beer requires a certain level of carbonation. For instance, German Hefeweizen, Lambic, and Saison call for about 4 volumes of CO2 (possibly more), whereas English Bitters should be carbonated to about 1 volume. In order to achieve a specific amount of carbonation, you're probably better off measuring and mixing up your own priming sugar yourself. How you would adapt a spray bottle to spray out the desired amount into every bottle is probably solvable (dilute or concentrate the store-bought sugar mix to distribute the appropriate amount per spray), but I'm guessing not as practical as measuring your own priming sugar according to the style of beer you're making and mixing it in to the entire volume of beer you wish to prime.
For me, I still think using an online priming sugar calculator (which also will give you the recommended level of carbonation per beer style), and adding that to the volume of beer being primed is the best way to go when bottle priming.