The other ways to do it mechanically are to use a switch (multiple switches actually) that the shift lever would contact based on the direction it's moving. A strain gauge inherently measures strain, so you can bond one to a shift lever and measure strain when the shift lever bends (when force is applied). That's how strain gauges attached to suspension members or driveshafts are normally used. You'd probably want to make a shift lever with a little bit of compliance in it just to make things a little easier, but that type of bonded-on strain gauge is a lot less straightforward to design and install than the pre-packaged load cell shifter setups so I wouldn't consider them.

Normally you'd interface the strain gauge with the ECU, and usually the strain gauge would need it's own electronics for amplification which would then connect to an ECU input. If it's a flexible motorsport ECU, you can put the voltage thresholds into the ECU calibration, along with any logic that would be needed to do what you need to. For example, you could have different strain thresholds for different gears, or have different strain values depending on whether you were downshifting or upshifting. That's pretty easy with a sequential transmission because you might already have a position encoder or potentiometer on the shift selector drum so the ECU can detect the gear position, or be able to detect if you were moving the shift lever forward or backward.
An ECU that can do gear detection can usually be programmed to change the amount of ignition energy cut depending on the gear or the throttle position (which is a rough approximation of engine load) or RPM. So you'd set it up to have a certain ignition energy reduction based on the strain threshold (input signal voltage), RPM, throttle position, gear, etc. It's not just one fixed amount of cut energy, it would obviously have to be a vector or a map if you wanted to take those parameters into account.

The Ferrari, etc., paddle shift transmission controllers have entire 3D maps to change the ignition energy, ignition timing, and fuel depending on those parameters, because those parameters have a huge effect on shift harshness (NVH, jerk, etc) and drivetrain life. That's all the way at one extreme though, and not really necessary for a track car with a dog box. You just need something to take the load off of the dogs (like you said) so it doesn't have to have lots of bells and whistles. Gear-based ignition cut is a very common thing to do with motorsport ECUs though, so many ECUs already have some functions built in right off the shelf.

Anyway, this is something that you need a decent ECU for if you have a fuel injected engine, or at least a fairly sophisticated ignition controller if you have a carb'd engine. There are some piggyback shift cut systems that go inline with the coil drive signal, but they're obviously limited in their sophistication compared to doing it directly with the ECU. Obviously the more signals you have access to, the more complex management strategy you can create.

If you're using the stock LSx ECU, you can probably find a piggyback that attaches in series with the 8 low voltage logic signals that go to the coils (those coils have the coil driver circuit built in IIRC) and reduces the duty cycle based on whatever parameters you can set. Basically the same thing those shitty Power Commanders do with the fuel injector signals on a motorcycle.

What are you using for an ECU? I haven't read back through the rest of this thread after the 1 year revival, so you might have already said..

I've done enough brain dump for today, I'm going to bed.