My inclination is in these circumstances is to try to embrace the facility you have rather than attempt to
cover it up. Strategically designed flats to
cover up backstage entrances works but I don't think I would
throw a curtain up unless there's a compelling artistic reason to. If anything, it would be flats around the perimeter of the space and dress the entrances up to look like they belong.
Audiences are very forgiving to the environment. If you
block the show so everything seems deliberate and designed for that space, then nobody will care that it's in a concert
hall. You can use gobos or projections to turn the acoustic paneling into scenery.
I wouldn't even worry about the exit sign. My preference is rather than to do scene changes in blackouts is to have the technical crew wear costumes and have each move happen strategically to lit to emphasize a different area of the
stage rather than the moving scenery around in the dark. High schools notoriously teach us that scene changes should happen in blackouts -- but blackouts with the sounds of shuffling and scenery clunking are really just awkward interruptions to the story that remind the audience what they're watching is fake, performed by actors and presented by technicians.
As for the
stage floor, it's not conducive to rolling scenery, but you can do a
ground cloth where you
lay muslin out in your scene shop or other open area (not on your nice wooden floor obviously) and paint it. Then you tape the bejeezus out of it to the
stage floor so it's taut and gives you a stable, painted walking surface.
Ultimately you can go a long way through the use of
props, flats, and very strategic lighting. What you choose to light, not light, or suggest a different reality to through the use of gobos that obscure it's a concert
hall can go a long way.
Unless of course the acting is bad and the
blocking is awful. Then go full-tilt theatrical because the audience won't forgive
jack if they're not consumed by the actual performance.
The best advice I can give you is..."Design for the space you have -- not the space you wish you had."