[ssrs-2008] Empty or Null value display in SSRS text boxes

While probably not any better than your solution, you could adjust your T-SQL to return the same result using COALESCE:

SELECT MyField = COALESCE(table.MyField, " NA")

The reasoning for the extra space before the NA is to allow sorting to place the NA results at the top. Since your data may vary, that may not be a great option.