It took me a couple of days (the whole weekend minus time for watching livestreams) and a lot of effort (I don't know PHP and generally can't write anything more complex than simple scripts), but it seems to work. The result is my previous post.
The feather (extention for Chyrp blog CMS that provides new format for your posts) is based on the default Photo feather, except there's no Alt-Text field. To choose several local images just hold Shift/Ctrl when selecting files. Remote uploading is also supported (didn't test it yet), just fill one URL per line in “From URL?” field.
Feel free to use it and fix bugs/extend functionality, because, as usual, if it works for me, I won't change it.