I think you misunderstand a few things. There is a reason that mtab has more information than /proc/mounts, and it is not that the kernel is stingy. The “user=” feature is implemented completely in userspace. In particular, that information never travels to the kernel via the mount(2) system call, but is enforced by the SUID-root program umount. Besides the “user=” information, there is also information pertaining to loopback devices that ensures proper unmounting of those as well.
So that information is written to mtab by the mount utility with the sole intention of its use by the umount utility.
If you want to know, the procfile output is controlled by the show_vfsmnt()
function. In order for you to change the current state of affairs, however, I believe you will have to ammend the sys_mount() and sys_umount() functions (but I may be wrong about that).