I ride my 9 year old gelding in a halter (rope) he is very responsive to it. I only ride him in the arena (haven't ventured out yet) but I don't think that would cause too much of a problem if/when we do. He too was an ex-lesson/trail/camp horse and my fiance and I felt the need to give him a break from the bit, and in turn we saw the Gem much preferred going bitless.
I did ride my (at the time) 4 year old mare in the same rope halter and I just felt it was a diaster waiting to happen. We have not tried it since.
I also briefly rode my qh gelding with nothing on, but it was more for just a "lets chill and relax" then any actual work. I let him go where he wanted.
I did get a bosal for him but I have yet to use it. Saving that for the Spring.