How do I programmatically set default table properties for CKEditor?

Here you go. dialogDefinition event solves the problem:

CKEDITOR.on( 'dialogDefinition', function( ev ) {
    var dialogName = ev.data.name;
    var dialogDefinition = ev.data.definition;

    if ( dialogName == 'table' ) {
        var info = dialogDefinition.getContents( 'info' );

        info.get( 'txtWidth' )[ 'default' ] = '100%';       // Set default width to 100%
        info.get( 'txtBorder' )[ 'default' ] = '0';         // Set default border to 0
    }
});

CKEDITOR.replace( 'editor1' );

More to read:

• This official guide will help you playing with dialog API (also with devtools plugin).
• Devtools plugin is helpful when looking for IDs and elements in CKEditor dialogs.

Have fun!