Do you know if hashtags put into the alternative text show up in tag searches. It feels like a much tidier place to put tags alongside the explanation of the image.

In some searches they probably can but that's also not a tidier setup. Voice readers will still say it. So you'd offload that extra wording to people with disabilities to make able-bodied people's experience nicer... Until bluesky maybe creates a septate hashtag field, best not to do that