The story goes that Johnson was putting pressure on the WC to issue their report well ahead of the 1964 presidential election so that the public could see in the report that Johnson was not involved in the assassination of JFK. This put pressure on the WC to get things wrapped up very quick.
However, to get around this issue with Johnson, the WC could just have issued an interim report in the summer of 1964 stating the basic findings that ended up in the WC report that there was no evidence that anyone other than Oswald was involved. The WC could have then taken the next 6 months or so (perhaps into the spring of 1965) before issuing the final report itself.
I don't think there is any good reason why Earl Warren, who was used to writing reports and interim reports, could not have simply followed this route.