Directly acessing the NULL bitmap of a row in SQL Server
No, there isn't. Besides, this is a very very very misguided effort. Since you've already loaded the page in memory, you have already paid the price for any row and any column on that page. This is a given for any row oriented storage. The price for fetching the page overwhelms the price of looking up values in the page.

If you want to optimize for retrieving the value (including whether it is NULL) for only a specific column then what you are asking for is columnar oriented storage, which is an entirely different game.