I too am trying to understand what you are trying to do.
A. Cutting an upright into two separate piece along the existing dado, using the dado's as a guide...
B. Making the existing dados deeper.
The way you were describing: "Doing a rough cut with a jigsaw..." he using a flush trim bit... It sounded like "A". ... And you were asking for something somewhat idiot proof.
Without outside support, using the existing dado as a guide, there is the risk of rounding the beginning and end of the cut... As you try to move the router to the dado with the bearing and as you exit the existing dado and lose that support. As we say, you ease it in and are careful as you exit. For most of us, that is good enough.
You asked for something "idiot proof." Some may say this following method as overkill, but you asked...
I would use a straight edged guide, just a bit longer than each edge, clamped even with the edge of the dado. Use it as the guide instead of using the existing dado.
With a guide, there is support for a confidently getting straight edge, including where the cut will start (before the material starts) and end, where the absence of existing material would not be able to support that (after the material ends)...
"Don't worry, I saw this work in a cartoon once."
"Usually learning skills and tooling involves a progression of logical steps."
Last edited by MAFoElffen; 11-06-2013 at 11:30 AM.