Well I guess your only options are either floating plants or plants that will grow on the bogwood in the absence of a substrate, duckweed is fast growing floating plant, another floating plant is frogbit, I think this is also a fast grower. Riccia also grows fast, this plant is usually tied to slate or rocks etc as it has no roots, I suppose you could tie this to the bogwood and it would look ok, you could also float it, Riccia will require decent amounts of light though, more than is usually supplied with kit aquariums.
I think most plants that grow attached to bogwood/rock etc would do in your tank, however they are not usually fast growers. Floating plants would seem to be the only other option.
Something like Water Lettuce would do the job and I find them far easier to control than some others like duckweed or riccia. Water Lettuce also develop quite impressive root systems that hand down from the surface ..... many types of fish (especially bubble nesters) love them.