I gave a bone editor a whirl, and found that I couldn't just load images for bones because:
a) I need to know the number of frames ahead of time
b) I need to know the number of different bones ahead of time.
My solution was creating canvases and pasting a "loader" sprite on them, which had a single frame loaded from the selected file.
It worked, no animation support but for animation editing I guess it would do. Size was all wrong and stretchy though, I have to try it with the latest build.