No it isn't going to work.
1. The water lettuce needs the water to enter below the surface.
2. The media needs positive flow through it; just sitting in the water will not get the job done.
3. The buckets will act as a restriction. Water will take the path of least resistance, which will mean going around the buckets.
4. The filter fiber will not work unless the water is flowing directly through it and has no where to escape except by going through it. Otherwise, as soon as it starts to get dirty the water will start going out the first holes it comes to.
What you should do:
1. Stack the buckets and have the top buckets drilled only on the bottom. This will guarantee flow through the mechanical filter (fiber).
2. The lower buckets can be drilled on the sides. Put scrubbies in them.
3. Elevate the buckets so that the rims of the top buckets are slightly above the water line. This will force the water in a downward direction through the media.
4. Have your return pipe teed to divide the water between the top buckets.
5. Place the water lettuce between the top buckets.