QML reset dialog with tabview

I am quite unsure, if I got your question right as you say, you can not reference the Dialog from within the Combobox. I can not see the reason why.

Assuming the example of yours contains indeed your problem and all you want to do is to reset the values (and you know the original values) once the reset button is pressed, this is how I would solve it.
Using the Connections-type to connect to the Dialog's reset() from within the Combobox

import QtQuick 2.3
import QtQuick.Controls 1.4
import QtQuick.Dialogs 1.2
import QtQuick.Layouts 1.1

Dialog {
    id: dlg
    title: "Settings"
    visible: true
    standardButtons: StandardButton.Apply | StandardButton.Reset
    property string val: ""
    onApply: console.log(val)
    onReset: {
        // **DONT** RESET COMBOBOX TO DEFAULT **HERE**
    }
    TabView {
        id: tabView
        anchors.fill: parent
        Tab {
            title: "ValueTab"
            id: tabVal
            GridLayout {
                id: gridVal
                anchors.fill: parent
                GroupBox {
                    title: qsTr("Choose value")
                    id: gb
                    Layout.fillWidth: true
                    ColumnLayout {
                        anchors.fill: parent
                        id: cl
                        ComboBox {
                            id: valueChooser
                            editable: false
                            model: ListModel {
                                id: listModel
                                ListElement { text: "One" }
                                ListElement { text: "Two" }
                                ListElement { text: "Three" }
                            }
                            Layout.fillWidth: true
                            onCurrentTextChanged : val = currentText

                            /// *** INTERESTING PART HERE! ***
                            Connections {
                                target: dlg
                                onReset: {
                                    // RESET COMBOBOX TO DEFAULT **HERE** INSTEAD
                                    valueChooser.currentIndex = 0
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}