Sorry for the misunderstanding. A straight line tool as you suggest may be a good item. I would have a question about how to keep the tool from lifting soft fibered wood out in uncontrolled chips at the junction of the border and lines end. I know that in engraving it is possible because metal structure is more dense.
An alternate method of bring the ends to depth is to use a veiner. James Tucker uses this method and is known for his work of excellence in this regard. It takes a VERY sharp veiner and a different angle at the tip to keep from having over runs...but it works. I have altered some bent files with blunted ends (60 and 90 degree) to accomplish the same thing. I pull them instead of push them at the ends so as to not leave overruns. I also have altered the guide on my MMC tool to allow the curved cutter to move further forward on my cut so that I don't have so far to bring to final depth.
