So I've schooled Snickers in a bit for two days in a row. Yesterday I just offered him to take the bit and, when he did that well, lead him around and reminded him the basics of bit pressure from the ground and then mounted. We first just walked on a loose rein and then on a very light contact, then trotted a bit, letting him get accostumed to the feel, but, when we transitioned down to walking again (all steering and transitions done with body only), he suddenly got very reluctant to move and started doing mini rears under me, that grew bigger with every offer to move a little. I got help to lead us around a little, then got a single step forwards without leading, got off and lunged him a little. There was no resistance while lunging, so I figured I got too tense myself, because I'm not really used to the thought of a bit. Took of the bridle, put on a cordeo and got on for a little relaxation ride. First Snickers was still in the error mode and offered me some smallish rears, but I managed to pull him sideways every time and soon we were trotting and cantering again nicely.
Today was much better. He seems to have started accepting the bit already and, although he still messes around in his mouth with it a lot, there are more and more moments with him going for the contact, which I've started offering lightly. We walked in trotted not only in straight lines, but also in circles and figure eights, and I got him flexing nicely along the long wall of the arena for a couple of seconds per try. I'll now get back to the hackamore for a while, but we seem to have made a nice progress in the "Operation: Bit" - at least, regarding his history with being abused with a bit by his previous owners and then just left at this stage of automatic resistance.
I have come a long way, to surrender my shadow to the shadow of my horse.