I found myself in a similar position when I was bottling my most recent pumpkin beer. I had brewed two pumpkin beers before, both which ended up with varying intensities of flavor. So for my third, I wanted to ensure a level of flavor I was happy with - so I made a tincture of spices and vodka and let it sit for a week (you can probably get away with less), shaking every day. Come bottling day, I added small amounts of the vodka-infused spices to the brew, stirred with a sanitized spoon, and tasted. Then repeated until I hit the ideal flavor profile, and added a tiny bit extra for insurance for the time it will sit in bottles (~8weeks+? it's a 9% abv)
This gave me a ton of control, much more than guessing how much aromatics would disappear after boiling / fermenting / secondary / etc. Plus, if the flavor begins to fade more than you would like, you can always add a bit more to the keg.
(disclaimer: as you can tell I, I bottle rather than keg, so I'm making the assumption it'll work for your setup.. but can't be sure)