Thanks for posting the follow-up.
16-bit words in a "B" file in a classic A-B controller (PLC-5,SLC-500 or MicroLogix) are treated just like words in an "N" file and are readable in 16-bit elements.
The reason that BOOL elements and individual bits of any B or N file cannot be addressed by a MSG instruction is a practical one based on technical support experience.
The theory is that if you allow programmers to read individual bits with a MSG instruction, they will be tempted to actually do so, and will use up all the memory and buffers in the controller moving around a handful of bits one at a time.
So Rockwell simply doesn't allow it, forcing programmers to at least move sixteen bits at a time.