Extra Order Items Modals

There should be a clean way to add our own order items metabox modals. Currently I either have to take over the entire template (which is difficult in itself, and a conversation for another time), or use a filter that puts the modal in a sub-optimal location. It would be cleaner if we could group the modals together in the same location, like this.

Thanks,
loushou

p.s. - this may also be bunched with the conversation in #6306, and if that is the case, just link them, and let me know.