Implementing fully immutable files

The man page on my system at least does note this under BUGS AND LIMITATIONS:
"Setting 'a' and 'i' attributes will not affect the ability to write to already existing file descriptors."

Some other attributes are also noted to not be implemented yet.

Also, even for chmod(2) isn't it filesystem-dependent what happens to already open file descriptors if a file is made read-only for eg? The manpage notes that NFS may apply that permission immediately to open files, and presumably local filesystems wouldn't.