Accessing Eager Loaded Transforms
It appears that the correct way to do it is using the url() method. I couldn't replicate the extra queries I mentioned in the question, and can confirm with some more testing that it doesn't cause any extra queries.

{{ teaserImage.url("transform") }}


or

{{ teaserImage.getUrl("transform") }}


Source: https://straightupcraft.com/articles/examples-of-eager-loading-elements-in-twig-and-php