I'm using rclone to upload a large file (~60Gb) to B2 storage. I also wanted to upload a custom header but it didn't work. It's worked fine if my file is small.
rclone --config rclone.conf --bwlimit 50M header-upload "X-Bz-Info-Uncompressed-Size: 10000000000" copy my-large-file "b2:my-bucket/"
I think if the file is too large, rclone will use b2-upload-part API instead of b2-upload-file for uploading and then this API didn't support custom header