With regard to water-based or aquaculture situations, I have been thinking a lot of duckweed (which you mentioned) in this regard (see http://www.fao.org/ag/AGAinfo/resources/documents/DW/Dw2.htm and http://www.wrc.org.za/archives/watersa%20archive/2004/Jan-04/16.pdf),
but I have not yet done research on potential problems of duckweed as a biogas substrate. Algae, in this regard, can provide some problems because unless the cell membrane is compromised, a good portion of it may go through the digester essentially intact. Duckweed is only about 7% dry matter (i.e. 93% water), so this may provide a challenge, since everything about biogas-- including, for example, heating systems-- has to scale with increasing dilution of the slurry. In either case, albeit more probably with duckweed, it may be that the plant could serve as
fodder, then putting the livestock manure through a digester.
An interesting comparison of duckweed and algae is found at http://library.wur.nl/wda/dissertations/dis3362.pdf (There is really quite a bit of good stuff available on the web about duckweed.)
Water hyacinth appears to be another promising candidate.
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