I don't accept that the revelation made to David was canonized Scripture. It was an event that was subsequently recorded in Scripture, but nevertheless, David did not draw that guidance from the Scriptures then extant; rather, he received this gudiance miraculously.
I think it's a question of whether or not the Bible truly is sufficient for guidance in every moral decision the man of God must make. The answer to that has to be a resounding YES. The difficulty arises when it is no longer obvious whether or not the decision is purely a moral one; certainly, the case with David's military stratagem was purely a tactical rather than a moral issue.
Also, re the question of whether or not I can supply a Scripture reference to say that revelation is ongoing, I think arguing from silence can be dangerous. The Bible does not plainly say that special revelation has ceased, nor does it plainly say it continues. This is why we have such difficulties between cessationists and continuationists.
It is also interesting to note from David regarding the promise he received concerning his future kingship, that he did not deviate from the Law of God in order to bring the promise to fruition, i.e., he did not kill Saul when he had the chance. In light of the promise, therefore, he nonetheless did what God ordinarily required of him from the Word.
