In the pico the Riccia is tied to 2 pieces of lava rock behind the main stones, in Autumn Sunrise it was tied directly to the main stones.
I think the trick is not to tie it too tightly, otherwise no new plant can develop under the fishing line, and it will not stay anchored. Also if the plant gets crushed in the process, most of it will rot and form a layer of sludge under the healthy stuff causing it to lift.
Use loads of line and tie lots of Riccia loosely to the rock, then prune it weekly to about 1.5cm thick, it can stay healthy and well anchored for 6 months or so.
Cheers,
James