One additional refinement: we may be able to use JS as an additional way to reveal (a tiny "click to reveal" icon or link?). There seem to be accessibility implications with having to click and drag, or it may just be difficult for people on some systems.

The only issue I can think of now are sighted people using text-based browsers; and the only way around that is to store the text somewhere else, which... would interfere with interfaces which can't use JS.