In our case, we can’t because we’re using the official topic thumbnail component which requires a local image. We solved the issue by adding newlines before any <img> in the content before topic being created with WP-Discourse. Not a solution for everyone but it works for us. A bit sad Discourse doesn’t support this legal usage.
But yes, if you are not tightened to a plugin/component and/or can’t fix the content before topic is created, this is for sure a reasonable workaround.