I've used peroxide undilluted to spot treat plants already in the aquarium and to sterilize substrate taken and recycled from a clado infested tank. Never as a plant dip though.
You've had plants survive the undilluted dip, so evidently, yes, it can be used undilluted. But some plants didn't take well to it.
Searching around, I'm finding people using various ratios for various lengths of time... Longer soaks for more dillute solutions. Which really doesn't clarify much of anything, and it's sort of hard to verify their techniques are working and their lack of clado, etc isn't just coincidental.
Doing some searching for you I found this thread... http://www.aquaticplantcentral.com/f...orn-algae.html
In it, HeyPK advises:
Once you have gotten your tanks free of clado, it is not too hard to keep it out. I examine all new plants with a magnifying glass. If I see any clado. the plant has to get a four minute bleach treatment. Fortunately, I find clado. attached to plants with thick rhizomes or to the older parts of crown plants. These kinds of plants can survive 4 minutes easily. With stem plants it may be attached to very old stems, which can be removed and thrown away, but never to newer, recently grown stems. Any other kind of hair algae found on new plants can be eliminated with a 2 minute treatment.
As far as the plants surviving, sort of hard to say for certain... Depends on the extent of damage, etc. Certainly couldn't hurt to try though... I'd give the sword a chance if nothing else...
Hope this helps.