The MK Hub redo is supposed to include access to two backstage routes from the Hub to Town Square, one on either side of Main Street. One is currently used in certain situations, the other I've never known to be open to guests. How much theming will be included in these routes is the OP's question and seems to me rather hard to answer until much closer to completion.
Do we have an estimated completion date on at of this?
The East side bypass is being worked on now and will be a street "themed" area. The West bypass (Casey / Crystal area) is used several times a year but recently, the last couple off weeks, has been opened on Saturday nights on a testing basis. Saturday nights this time of year have been very busy because of the party schedules. Currently no work or, AFAIK, no plans for theming for it.
They used the Casey/Crystal area on Wednesday 10/29 for exit during the holographic show and wishes. Was thrilled to see it as an option, and excited that it will continue to be so.
Several photos here, towards bottom.
http://www.orlandoparksnews.com/2014/10/magic-kingdom-hub-update-main-street.html
We were there Saturday Oct 25th and they used the bypass next to crystal palace after the 9pm parade-they actually had chairs set up in the bypass area to stay and watch the fireworks-it was really odd to see the backstage area but definitely alleviated the crowds-the bypass lets you put next to city hall in case anyone was wondering. I have a couple pics on my phone but don't know how to post them through the app lol