We have used 'mixed-policy MPU' with some buckets i.e. the main bucket is 3-replica but we manually create the +segments bucket as EC policy. (Note there is a bug fix needed
https://bugs.launchpad.net/swift/+bug/2038459 fixed by commit 60c04f1, so you'll need to be on a recent release). This isn't quite as flexible as what you've described.
This would be a nice-to-have feature for the new
native-MPUs feature where the segments/parts buckets are hidden, if we could dynamically create per-policy 'parts' buckets (embed the policy index in the hidden container name) and have the MPU manifest point to the correct parts bucket. One challenge will be figuring out how to represent the policy for the parts container with an S3 CreateMultipartUpload.