Typically, modules that use FileBackend will want to read from the closest cluster and write to the master cluster (or all, depending on implementation). The 'latest' flag could force reads to use the master (or cross-DC quorum read ect).
One way to due this is with a ReplicatedFileBackend class that works just like ReplicatedBagOStuff, using the 'latest' flag in the same way as BagOStuff::READ_LATEST. This works if the master cluster is in the one DC, which the other cluster(s) replicated from it.
A Swift setup using a global cluster spread over the DCs (https://swiftstack.com/blog/2013/07/02/swift-1-9-0-release/) would not use this setup. Instead, callers could just use SwiftFileBackend directly, which proper read/write affinity settings set in the swift proxy server config.
Additionally, deferring writes to the non-master backends should be a feature. This would handle updates for the 99% case, and a tool like swift-repl can fix any missed updates to assure consistency within hours.