As always call your region secretary, but the answer will be they are allowed. Someone mentions sight-reading below. It's actually a gray area I was told. The best solution in the sight-reading room is they can have them on their stands during the warm-up and then put them below their chairs afterwards.

Just please remember the sight-reading warm-up rules, I was part of a panel a couple years ago that a director didn't follow those and had to be warned. (additional instruction is not allowed, and it's not a full-on pre-performance warm-up!) Honestly I think if there's anywhere people will understand some tuning deficiencies it's going to be the sight-reading room since you left a warm-up room to a separate stage, and are now in a completely new room/temperature after not playing for at least 7 minutes of instruction time!