Sort entries by custom date in Matrix field / Super Table
I did something like this as a workaround:
Loop over entries, fetching all data you need and adding it to an array. Sorting that array with SuperSort and than displaying all info by looping over the array.

{% set tourDates = [] %}
{% set artists = craft.entries.section('artists') %}
{% for artist in artists %}

    {% if artist.tourDates %}
        {% for row in artist.tourDates %}
            {% set tourDates = tourDates|merge([ {
                'tourDate' : row.tourDate|date("d/m/Y")
            } ]) %}
        {% endfor %}
    {% endif %}

{% endfor %}

{% set tourDates = tourDates | supersort('sortAs', '{{ object.tourDate }}') %}

{% for tourDate in tourDates %}
    {{ tourDate.tourDate }}
{% endfor %}