Artists often upload temporary images on their pixiv accounts notifying their followers that they're streaming.
These images are often unfinished or cropped lower quality versions of older images with the notice written on them. (mostly below Danbooru standards)
Currently, it's mostly ニコ生 or ニコなま for Niconico streaming. Previously it was スティッカム配信中 (Stickam), 絵チャ (oekaki chat) or UST (UStream).
Regardless of what service they're using, I think it would beneficial to create one tag for all of those.
The niconico wiki currently states that you should use it when a notice appears, but I don't really like applying a copyright tag for some text that appears in the image.