Facing charges for driving on a suspended or revoked license: Is it too late to challenge the basis for the suspension or revocation?

It depends.  The defense of your current citation would not include the ability to challenge the validity of the suspension.  The officer just has to prove that you were suspended.  I have had cases in the past where we did what is called an appeal nunc pro tunc and attempted to reverse the suspension.  If you are successful often times you can get the officer to remove the driving while suspended ticket.  This is not an easy process and really should be done with the assistance of an experienced attorney.  You are talking about 3 or 4 court appearances and many times negotiations get you to a better result than actually trying the matter.  Sit down with a local traffic attorney and let the work through the details.