Although the question suggests and override case, it also aks for a case where the css or js are in a common folder, away from the bundle structure.
One answer is to use the available twig variables. For example if I have one css file located here
app/Resources/views/clientSite/customFolder/css/mycss.css
I can load in any template using something like this (note the example overrides the full stylesheets block, but not required, the stylesheet twig tag can be added in any block):
{% block stylesheets %}
{{ parent() }}
{% stylesheets '%kernel.root_dir%/Resources/views/clientSite/customFolder/css/mycss.css'
%}
<link rel="stylesheet" type="text/css" href="{{ asset_url }}" />
{% endstylesheets %}
{% endblock %}
Also remember to execute the assetic:dump command, so symfony knows it had to publish the css to the web/css folder with all the other files.