Any way to manually generate a transform on an asset source?
You could just make a temporary template that makes Craft generate them for you. And depending on how many these are, you better use limit and offset with your craft.assets call.

{% set images = craft.assets.source('myAssetSource').limit(50).offset(0) %}
{% for image in images %}
    &lt;img src="{{ image.url('fullwidth') }}"&gt;
{% endfor %}