For the member halls - yes - just request it and pay the extra fee.
You can ask for a table next to a friend if both registrations are sent together.
You can state a preference for one of the member halls, but no guarantees. I prefer to just wait and see where I end up. Over the years I have had successful sales wherever I was. It's more a matter of your pricing of items than location IMO.
I had my best York ever when stuck in an out-of-the-way corner of the old Black hall, back in the days when it was used for sales.
If you have interesting merchandise and realistic prices, they will find you.
Jim