How do you know that the problem is the mandatory lock support?
I guess you did not switch it on (it's a mount option).
> Most notably, 'sendmail'+'procmail' is not interlocking with
> 'movemail', causing entries in /var/spool/mail/ to be truncated
> mid-message, with ensuing mail lossage.
Are you sure that procmail and movemail (and all other mail readers) are
using the same locking strategy? (flock / fcntl / dotlock files)