The video wall was there until not that long ago really, I think it was only around 2010/2011 they got rid of it, which disapointed me! I've been going to Meadowhall since the mid-90s though, and I don't remember them ever simulasting YTV, so that must have stopped early on. They used to run all sorts, not just promotional stuff but music videos and cartoons and stuff too.
I was talking about very early on, you might get a bit of The Time The Place or whatever early in a morning, but I doubt it was ever intended as a proper sustaining service - it may not even have been intentional given the implications of such a simulcast. I just recall whenever I walked through first thing in a morning, there'd be GMTV or the early YTV lineup on there, with the proper promotions stuff really kicking in about 10. Perhaps it was just used as a TV for staff until the big rushes started back then?
In addition, I don't recall seeing Meadowhall themselves using it that much, aside from the playout, it was pretty much YTVs base for down the line/ small pickups, much like the upstairs of Radio Sheffield is for the BBC.