Methods
First a tedious but accurate method for adjusting hop attributes of finsihed brews:

1. Taste brew and determine what hop attribute(s) are needed.
2. Adjust water to pH=4 or so.  This gets it in the range of most finished brews and is said this minimizes leaching tannins.  I use diluted phosphoric acid.
3. Bring water to boil.  I use a 8 qt. pot.  The bigger in diameter, the better- see 7 below.    .
4. If more aroma than flavor is needed, kill heat, pitch hops, cover, and let cool.
5. If more hop flavor than aroma is needed, pitch hops and boil for 5-10 mins., kill heat, cover, and let cool.
6. If both hop aroma and flavor are needed, do a combination both of the above steps.
7. Rack to clear bottle or jug.  I use only whole hops so I whirlpool.  After cooling to settle them in the middle of the pot and then insert a circular manifold a bit less in diameter than the pot and made of 3/8" copper tubing with lots of small holes to rack the tea.  A scrubbie over a racking cane would also work.
8. Take a measured sample of the brew, add a measured volume of the tea (a syringe is nice ideal for this) and taste/smell the result.
9. Repeat the above step until satisfied.
10. If adding bitterness also, add  Iso-Alpha Acid Extract from HopTech  (teas don't add much bitterness) to the tea and repeat taste testing above but add the volume of tea you've determined needed for flavor and/or aroma   If more bitterness is needed, add more extract to tea and repeat.
11. Scale up the ratio of tea-to-brew you've determined via taste/smell testing to the volume of brew and add the calculated amount of tea to the brew.
12. Taste the result and tweak by adding more tea if needed.

Since the usual problem with my brews is not enough hop aroma, I usually simply make an aroma tea per 1 through 4 above and pitch the whole amount into the brew without any taste/smell testing.

Tips/Other
I've found that a bit of the aroma from teas is lost in the finished brew so, when adding for only aroma increase, I add more tea than I've determined needed by taste/smell tasting.

I usually end up with around 4.5 gallons of kegged brew, so I use at least 1 qts. of water for making the teas and at least 1 oz. of hops. The volume also depends on how much dilution the brew can stand- or perhaps even needs.

I've not tried them, but HopTech also sells oils for adding flavor and aroma which might allow you to forego having to make a tea.

Rack the tea to a "minikeg" rig made from a 3L plastic pop bottle (details).  Here's the set-up:

Open the valve in the brew line, tweak open the keg's co2 vent valve (a needle valve if great for this!) and adjust until there's a very slow flow fo tea into the keg.  One can raise or lower the  minikeg in relation to the keg elevation to further tweak the flow.   If transfer is too fast, the brew will loose carbonation and you may get foam out the co2 vent valve.

If the brew in the keg  was naturally carbonated or you have cold chill or other sediment in the keg, the above method will likely cloud your brew.via the flow of tea from the keg's dip tube stirring up the sediment.  Instead of the above mehtod, maybe try slowly depressurizing the keg and adding the tea via racking through the keg's co2 disconnect/keg's releif valve opened or via the opened lid.  I've not tried this because 'cause I worry about foaming as the tea is added and then having to carbonate the keg again.