Is it possible to make the yesno filter return a single comma (",") on true as it is also used as delimiter ({{foo.bar|yesno:"yup,nope"}}
~> instead of "yup" return a comma)?
Just as an alternative to the custom filter solution already suggested, you could HTML-encode the comma like so:
{{ foo.bar|yesno:",,nope"|safe }}
If you're reusing this logic in many templates then a filter is definitely more maintainable.