So, I went hunting through the XML and found where Jaywalker is defined. All I can tell about the frequency with which it should appear is that it's part of the "Incredibilities" item pool, which I guess means you wouldn't see it very often. In fact, the only time I ever saw any of these was in a shop where I had just killed the shopkeeper. So is that the only way you get Incredibilities offered to you, or do they ever appear as pickups (like in a loot chest or on clearing a room), or do they get offered as level-up perks?
Also, when I visit a robot shop, shouldn't I consistently see different robot options offered the first time I go into the shop on any given floor? Naturally, once the shop's been entered the first time, the offerings become static, but shouldn't they change on a new floor? In the run I just finished, every robot shop offered the same exact selection.