I never actually tried to have a nano filterless planted tank on purpose, but I have had a 1.5 gallon acrylic critter box with 3 blue shrimp (neocaridina, but they are actually not blue http://www.petshrimp.com/neocaridinaspblue.html
) fliterless for over 3 months already.
They 3 shrimp were mixed with a batch of cherry shrimp I brought, and because I didn't want the 2 to cross breed, I separated out the 3 blue shrimp in the 1.5 gallon tank without any filer. (It was intended to be a temporary hosing until i figure out where to put them....)
I threw some flourite substrate, a clump of java moss, and a bunch of excess riccia i had in there, and pretty much just left it like this without filter since March 2008. Never had water change, only top off the water every 2 weeks or so, and it gets 23watts CF light 10 hrs / day.
The shrimp are all doing fine, the riccia covered the entire top, and the java moss probably tripled in size.
So...i'd say it can definitely be done if you have enough lighting and plant density.