After more than one year, the solution finally comes! :-o
http://www.metz.de/en/flash-units/firmware-download-flash-units/mecablitz-50-af-1-digital/pentax.html
This is for the Mecablitz model 50 AF-1 for instance. New firmwares for other current major metz models are also available, which is the solution to enable the High Speed Sync when using their flashes on the K-30 or a later Pentax DSLR body.
Update (6-23): Taiwan Pentax K-5 users have reported that after upgrading to the latest version 4.0 of the Metz firmware, their flashes (model 58 AF-2) have started not to fire in both A and M modes! So, if you are a K-5 user, do note the issue and should not upgrade! This story once has reminded us one thing, i.e., if you do not *really need* an upgrade of any firmware, just do not do that, as the latest is not always better. For instance, it is just a matter of compatibility after all! :-o