>>>>>>perhaps Paul uses the term as we might use the term "comrade" . . . <<<<<<

I believe this can be ruled out. (For one thing, this James was not a follower of Jesus during his earthly ministry, so they were never 'brothers in arms.')
Catholics maintain that Mary had no other children but Jesus. (Joseph might have had children by a previous marriage, a view championed by some in the early Church.) There *is* wiggle-room in the use of "brother" in the Bible because among Hebrews the term could refer to blood brothers or more distant kin.
The problem with this is that the NT was written in Greek and Greek clearly distinguishes between the two. (Elizabth, the mother of John the Baptist, is referred to as a "kinswoman" of Mary but never as a "sister," for example.)
Most Protestant scholars hold that although "brothers" can have a wide sense----as in "brethren," a term Paul used in some letters---it seems clear that Paul is calling James----and no one else in the NT is designated this way---"the brother of the Lord" in the sense of sharing parents (or at least one parent).

I think the natural reading here is the best one. Again, Catholics don't care for this reading (-unless James is seen as a child of Joseph but not Mary) but nothing in the Bible supports the alternative reading, and all desire *to* read it that way comes from a time after the death both parents, James himself, and any other siblings he may have had. (I'm not sure James was married.)

Nolen, the terminal I'm on forbids access to the link you posted.

As for the dating, it's hard to imagine this being written *after* the destruction of the Temple in AD 70. Anytime before then would have James in the running, wouldn't it?

There's a book on James by Richard Bauckham that I've been meaning to obtain through inter-library loan. I guess "now's the time."