Congratulations you just became a grass farmer! It is time to take a serious look into pastured grass fed beef my friend.
So based on the following description:
Phytoremediation and biomass
Reed canary grass grows well on poor soils and contaminated industrial sites, and researchers at Teesside University's Contaminated Land & Water Centre have suggested it as the ideal candidate for phytoremediation in improving soil quality and biodiversity at brownfield sites. The grass can also easily be turned into bricks or pellets for burning in biomass power stations.[1] from:
http://en.wikipedia.org/wiki/Reed_canary_grass Plus your own experience with comfrey and your willingness to try different things I would suggest working on building the best soil possible. My thought is the canary reed grass is working to bring back the soil (pioneer species like). Your experience with comfrey and the observations adunca has made regarding nettle plus knowing that both nettle and comfrey are great accumulators seems to me to be worth exploring.
A soil test would be high on my list to get started. It would be interesting to see if there is anything obviously out of whack (from the link below I would guess you have a high nitrogen load), it would also serve as a base line to measure against as you try different things moving forward.
This was an interesting read: www.botany.wisc.edu/zedler/images/Leaflet_5_nov_05.pdf and may change the below list up some depending on your conditions.
A few additional ideas for soil building and potential eradication:
1. Heavy mob graze the areas. The cattle love it and will help fertilize building soil fertility.
2. Run chickens through the area, would be good if they could follow the cattle. Chickens will scratch up just about anything given enough time. They would also be upping the fertility naturally for you. I am suggesting really pushing it here to near bare earth conditions with the chickens. If you do have a high nitrogen load make sure you have a lot of carbon in there with the chickens.
3. Mow the hell out of it. You might have to abandon your previous plantings until it is under control. I know it is tough but it is just a plant. Starve it enough and it will give up eventually. Mow before seed ever sets, mow on the hottest days. What you want to do is stress the plant into using the reserves to put out new grow and then mow that off too. Then if you can, run chickens in there after.
4. Brush piles and controlled burning in small veggie sized areas. A nice hot burn pile will heat the earth up pretty deep and should kill off the seeds and rhizomes beneath it.
5. Dewater the area! Willows would take time but should help.
6. Carbon material, wood chips, saw dust etc. Guessing on this again but a soil test will tell you yes or no here.
Good Luck and if you can/feel inclined post any results I would be interested anyway to see the out come either success or failure. Also if you get or have a soil test put that up too as well as your location/growing conditions etc. The more info we have the better especially in your case dealing with an invasive.
Jeff