In the current beta (beta release 1.51.0-124-ge569977c), when you try to delete a file from a union, it will try to delete the file from whichever upstream contains the file, even if the upstream is read-only. In the current stable release it will even delete files from upstreams that are explicitly stated in the docs to be read-only.
It would be great to support something similar to unionfs's whiteout feature, where a file is created in a read-write upstream that notes to unionfs that the corresponding read-only file should not be displayed. This would allow deleting from read-only upstreams without actually modifying them.
My personal usecase is generating "patches" between two different directories containing only the changed files, and I'd like to be able to pick out whiteout files to generate a list of all deleted files to include in the same patch.