To use a specific compression algorithm simply uncomment it in mkinitcpio.conf (and comment all the others).
For options, see COMPRESSION_OPTIONS array.
Important: Do not use the above as I think it applies to all compression algos.
From manpage:
This option is generally not used. It can be potentially dangerous and may cause invalid images to be generated without any sign of an error.
If you’re adventurous, you can modify /usr/bin/mkinitcpio like I already described here. I don’t support or endorse this 