We tried it yesterday, didn't go very well. When I was supposed to dissociate, suddenly the aliens appeared. They said everything's going to be OK and at first I wasn't afraid. However when my T suggested to "let them help me" I got into a bad and deep derealisation and was very scared.... I woke up several times at night and had a nightmare about a huge fire in my town.
This might be a useful technique but definitely not for people who suffer from too much dissociation.