I don't see powering the Dyna plate from the brake power as a big problem. The power comes from the same fuse and ignition switch contact. The coil power still comes from the kill switch. I don't think there's any risk of damage from the fact the plate is powered when the kill switch is off but ignition is on.
Running a long wire up to the coil area has a few problems. You have to splice on a longer wire, splices are prone to problems. The wire up to the coil area is itself open to damage. The coil power junction is likely the lowest voltage in the harness, since the coil current through the wire, connectors, etc. will produce it's maximum voltage drop there.