In trying to recover a filesystem hitting LU-14958 with very slow journal recovery, we were unable to run "tune2fs -f -E clear_mmp" to clear the superblock, because this was going to take about 30 hours to finish processing the journal revoke blocks. I wanted to use something like "debugfs -f -w -R 'set_mmp_value seq 1234' /dev/MDT" or "... 'feature ^mmp' ..." to reset the e2fsck sequence number, but it was not possible to open the filesystem as it continually returned an error when trying to open it in write mode:

# debugfs -f -w -r "set_mmp_value seq 1234" /dev/sdc
MMP: e2fsck being run while trying to open /dev/sdc