request for darcs to (optionally) exclude binary executable files
having it consider any executable file that does not begin with #! or contain a 
null in the first 4k would be a sufficient check

Steve, 

Since Darcs 1.0, darcs has automatically files as binary which contain ^Z or \0
(the null character). This covers a surprising number of changes. 

There is also a built-in system to declare which files are binary are not, which
you can read about in the manual:

http://www.darcs.net/manual/node5.html#SECTION00510000000000000000

As a result of your report, I have just now prepared a patch to the
documentation that clarifies the auto-binary detection. 

Since the functionality is already there, I'm considering this issue "resolved". 

  Mark