You might have some luck with microsword or marsilea minuta. I have heard of crypt parva doing the trick as well, although it is a super slow grower.
Your best bet might be to do a moss carpet, with moss tied to stainless steel mesh. I saw a tank journal where they did this not too long ago.
I would avoid plants like hc, glosso or hairgrass. Probably more trouble than they are worth.
People really have mixed results when they try to get a carpet with low tech.
Let us know what happens!