Yes, even if access is forbidden, nothing is closed, amazing because it could be dangerous, but if you want you can
I went inside the old building when the renovation began, just about 1 year ago, from the last floor just below the roof (pics on this same page, building with the grey circle which closed the hole since the old clock as been stolen!!), there's a nice view of the construction site and I took many pictures inside and outside.
At this time, there was no worker the week end, but with the Design biennal deadline... things changed.
Last sunday, there was only 2 people working on the cladding, they didn't speak french, only english and german and they just didn't care! so I could shoot the Platine from the inside.
Same easy access in the building at the beginning of this page, with the old factory chimney, a former administration building which has been renovated in very nice appartments, each ones on the last floor have roof terrace.
The Zénith is an exception, I tried many times but I didn't manage to get in. At the end of this week, and for 2 days, it will be opened to public, I hope I'll have time to go there and take some pics.