Generally seems fine, minor change requested.

Note, we could have the metadata drawing in ed_util_imbuf.c, since this function is closely related to imbuf data. - Other opinions?

\name Generic Callbacks for Drawcall API

There is no "Drawcall API" documented anywhere. I realise that this is just a restyling of the pre-existing comment, but still it makes me wonder what is meant by it. If something is named "API", I would expect an interface definition somewhere. In this case there isn't even an ed_draw.h file that exposes this interface. The ED_region_draw_mouse_line_cb() function is declared in source/blender/editors/include/ED_space_api.h; maybe that should move to a different file too?

Talking about ED_space_api.h, that now reads:

/* generic callbacks */
/* ed_util.c */
void ED_region_draw_mouse_line_cb(const struct bContext *C,
                                  struct ARegion *region,
                                  void *arg_info);

The "generic callbacks" comment is very weird, because it doesn't specify what actually accepts this callback function; it doesn't document what the function is for at all. It also doesn't explain why the ED_region_draw_cb_...() functions declared above it are not "generic" and this one is. This is all not strictly part of this patch, but still, now that you've looked at the code, understand what it does, and that it needs to be in a different place, you're the prime candidate to also clarify these functions' declarations.

What is part of this patch is that the comment stating it's implemented in ed_util.c is now incorrect. Personally I'd remove that comment, as you've proven that it's very easy to make it go out of date and it doesn't serve any practical purpose (any IDE worth its salt should make it easy to find the definition from the declaration, and otherwise a quick grep will find it for you).

I'm also not a fan of having one C file that has part of its interface declared in ED_space_api.h and another part in ED_screen.h.