command-line tweaking of "Open With..." settings?

I have an AppleScript Application which I wish to have launched
whenever a file named "*.xyz" is double-clicked.  I can do this
using the Finder's Get Info dialog (ie, "Open With...", then
"Other...", then "Change All..."), but I want to do this from an
installation shell script, so I need a "command line" solution.

I have tracked the setting down to the file

  /Library/Preferences/com.apple.LaunchServices.plist

but the contents of the setting that Get Info creates are rather
obscure (to me), so I don't know how to replicate them.

Clues? Suggestions?

-r
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