Their insurance will be sky high to have people attending events with steel going up between the events. I would not want to be sitting in that place with high winds and heavy snow with an incomplete steel structure over head.
The SU steam station shell along 81 fell over in a windstorm before it was completed. I worked on that and it was around 1990 when it happened. The dome is much taller and on a hill.
I have been on construction jobs were you can't enter certain areas until all steel has been tied in together. You have to tie the shell into the existing dome as the columns will have to be very,very tall to reach ever the fabric roof.
Work will not start until 2017 I was told just a few days ago again.
Here is the specs on he biggest mobile crane and one like this just fell over very recently in NYC. The boom on this crane just will not reach out far enough to do this project. The dome is wider and longer than this boom alone and when you figure in angles and reach,it ain't gonna happen.
http://www.popsci.com/scitech/article/2009-03/tallest-mobile-crane.
They will need to set up cranes like this from the inside to do what the want.