Get entered value of customiser field for live preview

Yes. wp.customize( 'header_textcolor' )():

( function( $ ) {

    //Update site background color...
    wp.customize( 'background_color', function( value ) {
        value.bind( function( newval ) {
            $('body').css('background-color', newval );
            var text_colour = wp.customize( 'header_textcolor' )();
            // ... now do something with text_colour
        } );
    } );

} )( jQuery );