Your best bet is to train your content editors to follow the content guidelines of entering the data correctly as per your needs.
We work with CE on a large scale efforts. Probably we have 10 CE just doing contribution on our multiple projects.
This can be easily handled by WYSIWYG by going to the source and add alt tag to it.
We have this on our content guidelines as we care deeply about accessibility. A little training will go a long way rather than finding programming solution for this.
Sharing through my experience I could be wrong too. That's how we addressed this after juggling for 4 years.