Tags would be good. Rather than crossposting, tag your post in up to 5 communities. A photo of a bird might be tagged in Nature Photography, Birds, Animals, and horses.
If a the moderator of c/horses decides the submission is not apropriate, they can un-tag themselves. If all communities un-tag themselves, the submission is ‘orphaned’ its no longer visible in any C, but still exists in the OPs profile.
The user cannot re-tag c/horses, but can add a new tag if they want. If our man keeps tagging birds as horses, then they might be banned from submitting to the horse tag for a period.
This allows one post to be visible over many communities without crossposting, reposting, or having related overheads of duplication.
If the user does not want to be harrased by ornithologists arguing over whether they think its a western red breasted great titwarbler or a northern red breasted lesser titwarbler, they may also untag their submission from c/birds
Tags on a submission would be visible to users, so if I find this cool titwarbler photo through the nature photography C, but I want to see more titwarblers, I might check out c/birds, or c/horses.
Some of the larger Cs might opt-out of the multiple tag system, allowing submissions only if theirs is the only C tagged.