Using variable references to fields in a template
Figured it out:

{% for n in [1, 2, 3, 4, 5, 6] %}
    {% if attribute(matrixBlock, 'quote' ~ n)|length %}
    &lt;blockquote&gt;
        {{ attribute(matrixBlock, 'quote' ~ n) }}
        &lt;cite&gt;{{ attribute(matrixBlock, 'attribution' ~ n) }}&lt;/cite&gt;
    &lt;/blockquote&gt;
    {% endif %}
{% endfor %}