I made some changes to MMP to push it upstream:
http://thread.gmane.org/gmane.comp.file-systems.ext4/24675

e2fsprogs patch:

• use blk64_t instead of blk_t
• bump update interval to 5s

kernel patch:

• add more comments to struct mmp_struct
• remove unused EXT4_MMP_UPDATE_INTERVAL
• change check interval multiplier from 5 to 2
• use ext4_fsblk_t instead of unsigned long for the MMP block

We should integrate those modifications into our own kernel & e2fsprogs patches.

Here is the initial thread for reference:
http://thread.gmane.org/gmane.comp.file-systems.ext4/24301/focus=24306