For no-hands, get up to a high wheel speed. This gives more stability to the whole setup.
Start practice with hands on the bar tops, close to the stem, and get comfortable with the minor inputs it takes to change position on the rollers. Work up to lifting one hand and making small, subtle changes to lean to control position. Then get up off both, but be ready to drop when the inevitable weaving begins.
This is all about making small corrections from a lean at that saddle, doing them early and as small as possible to keep the changes and counter-changes to a minimum.