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I've heard you can turn wisteria on its side and "carpet" it... Or there's always Vallisneria, it gives off runners as a makeshift "carpet" of reproduction. Not sure how T8 bulbs would do with these but they are considered low light.
Crypt parva or dwarf sag are probably the best you'll get to a low light carpet. Although dwarf sag is still a medium light plant and the crypts would take ages to fill in.