Ok, got this working. There was a bit of trial and error, but it works quite well now.
I created a water wheel with a gear & axle to provide power to the roller on the repeater track/loop.
I made a loop of track 13 tiles per side.
I placed a single roller on one side of the repeater track loop, speed setting LOW.
I powered the single roller with a gear next to it from the water wheel & axle.
Opposite the roller tile, on the other side of the loop, I built a pressure plate set to trigger on the track.
Then, on the same tile as the roller, I created a new route, and a new stop.
I removed all conditions for the route.
I assigned a vehicle (mine cart) to the route.
Then I built the upright spear trap.
I tied the pressure plate on the track loop to the upright spear trap.
I then built a RS|FG|PP|FG|ME as I mentioned above, and tied the water-triggered pressure plate from that to the gear powering the roller.
I then built a lever, tied it to the same gear powering the roller, and disengaged the gear with that lever.
Next, built the entrance-hallway pressure plate (creature triggered), tied to the floodgate next to the riversource, which the goblins will step on.
I also built a second pressure plate (citizen triggered) with a dummy lever, as a reset, tied to the second floodgate (the one next to the map-edge)
The sequence then goes:
Goblin steps on pressure plate at the end of the hallway, which opens floodgate1. This floods the pressure plate, which engages the power to the gear powering the roller. This engages the repeater, which oscillates the upright spikes forever, until a citizen is told to flip the dummy lever. Any number of goblins can walk over the plate, it doesn't matter, the spikes will continue oscillating without interruption, driven by the track loop repeater.
When all the goblins are dead, I send a citizen to flip the dummy lever, he walks over the citizen-sensitive pressure plate, which temporarily opens the second floodgate, unflooding the water-triggered pressure plate, ceasing the oscillations.
The hallway is now ready for more goblins, and will again engage automatically.
This was all done in 34.10.