Disappointed that no replies here or in Python forums so went with AppleScript application:
-- defaults write as an application to set mailto handler to Outlook
-- plist is under ~/Library/Preferences/
-- osascript -e 'id of app "Microsoft Outlook"'
-- will give you the com.microsoft.Outlook
do shell script "defaults write com.apple.LaunchServices/com.apple.launchservices.secure LSHandlers -array-add '{\\"LSHandlerPreferredVersions\\" = { \\"LSHandlerRoleAll\\" = \\"-\\"; };LSHandlerURLScheme=mailto;LSHandlerRoleAll=com.microsoft.Outlook;}'"
display dialog "Reboot required to take effect" buttons {"OK"} default button "OK"