I have considered adding a dose of "hop shot"... essentially pre-isomerized hop extract. You can get it from Northern Brewer in a small syringe dispensing tool.
My thought was to add the extract to some hot water to dissolve and then add to the final beer. It would add some hop flavor and bitterness. Because it is processed differently than a tea, it would not have the grassy flavors of boiling whole hops. Also, because the alpha acids are isomerized, they will dissolve in the beer, unlike the hop tea technique. (so it is more like the hop bitterness/flavor you get from kettle hops instead of dry hops)
I have the vial in my fridge, but have been fortunate that my beers appear to be dialed in now, so I have not tried out the theory.
The vial has marking on it, so you can dispense fractional portions of the tube with good precision.
The other thing I have done... is simply brew the beer again with a bumped up IBU number.... after experiencing low perceived bitterness on a number of beers, I learned that for my system... it is usually good to just bump up the IBU 5 points from standard recipies...and they all come out good now.