This is the solution (one solution at least) for including a file with the file path coming as a setting (e.g. from a profile).
These are the contents of the template file:
{% if config_get("environment.env", "") == "prod" %}
{% with filename=config_get("environment.templatepath", "")|add:"/prod.txt" %}
{% include filename %}
{% endwith %}
{% else %}
{% with filename=config_get("environment.templatepath", "")|add:"/test.txt" %}
{% include filename %}
{% endwith %}
{% endif %}
Great success!
Thanks!