when we work with a team of translators/content reviewers, it'd be very usefull to be able to have a "content mode" on front-end.
This would be also very practical when you check all your pages, and need to change just a label somewhere.
Is there any particular reason, why you just can't create single user only with site content editing right for all that group of people. You need some way to identify them in any case, since I don't suppose you wan't any site visitor to be able to switch to content mode?
I think about admin users who could have a special perm to see displaying these magic yellow boxes.
Phrase editing permission comes together with content block editing ability, which is checked by CATEGORY.MODIFY permission.
Additionnaly, what do you think about a system to make label translation as easy as displaying an input field when clicking the Edit button, without any pop-up/modal window?
Specially for users, who don't like popups we have nice feature in debug mode: all untranslated phrases are displayed as links, who will lead to phrase translation page in same window. You even don't have to go to admin, just enable debug mode for user ips who are interested.