Ha, I just posted about hit detection minutes ago:
Corona touch listeners on parts of a spine animation
You're on the right track with AABBs. Unfortunately, as you found, Spine only provides bounding boxes right now, which are arbitrary polygons. We did bounding boxes first, with the expectation that we'd add rectangles and circles soon after, but we got side tracked with so many other big things. I do think it's something we can do in v4.1. We're wrapping up the v4.0 release now, though it'll likely still be a few more weeks.
Until then, there may be workarounds that you find acceptable.
The simplest is to get a bone at runtime and do hit detection from that point, ie use it as the center of a circle or offset a rectangle from there. The downside is that you can't visualize it in the Spine editor.
You could use bounding boxes. They are polygons and the vertices don't currently allow exact/numeric positioning, but still bounding boxes with a low number of vertices (say 4-8) may work for your needs. They have an advantage that they can be deformed, either via bones or deform keys. Note deform keys are generally to be avoided. Deforming a bounding box likely means you don't need as many attachments, since the alternative would be to show different sized bounding box attachments as needed (or AABB or circle attachments).
Lastly, you could mimic AABBs by using a region attachment. For example, create a square image and create region attachments using it. Position these where you want your AABB hit boxes. You may want to put them on the root bone, so they are not transformed by other bones. Or you could create a bone that has inherit rotation and scale disabled, then attach the region attachments to that. This way you can have the AABB region attachments translated, but not rotated or scaled. At runtime, don't draw these attachments. To do hit detection, get all the visible AABB region attachments, compute the world position of one of their vertices, then add the width/height to get the other 3 vertices. We can help you with any of these steps if you like.
