--early on in my investigative process--but nothing came of it--clear as a bell, no cells, no signs of protein or infection.

There are a number of neurologic conditions in which there are specific signs that often show up in spinal fluid--often these are autoimmune in nature (like mutliple sclerosis and chronic inflammatory demyleinating ployneuropathy), so a spinal tap might well be warranted if there are not more obvious "smoking guns".