@siderea @Lilysea There is not a non-clunky way that I have found.
You can either grab the original link (not the link as it turns up from your instance, this is a massive pain) and copy and paste it into a new toot with a CW, or you can do a reply with the CW and some text like "please read the toot this is a reply to". Both require your readers to do extra work.
I get that the nonexistence of quote toots is for reasons, but I do think being able to add CWs to boosts would be really good.
@Lilysea No idea. I would ask the original poster, or maybe reply with an image description and then boost both the original and your reply.
I think it's broadly accepted that some people struggle with adding image descriptions, so you may be doing the OP a favour.