You can use Filesys::DiskSpace, the Perl df. See howto-write-perl-script-to-monitor-disk-space for ideas.
Drumming up your own solution with the Unix df command is only 10 lines of code or so. You could use something like sendmail to notify you if the disk space reaches a certain threshold. No experience with 'munin', scanning through the documentation it seems like everything but the kitchen sink.
But what can your ETL tooling do for you? The ETL tools I worked with also produced useful statistics. Things like throughput, disk space requirements etc. Maybe this is something to look into.