Have you tried using AI servo in low light? Wouldn't it have been nice to have your AF point illuminated in red so you can actually see what your focussing on? Without the illuminated AF points, AI servo would be crippled in low light for me.
There's no reason why this feature shouldn't trickle down to the 5D, or spot metering tied to AF points given the fact they share nearly identical AF systems.
You sound quite certain about that…I'm not so sure.
For AF point illumination in AI Servo, as I understand the issue, the problem is that the the light which illuminates the AF points also affects metering. In prior 1-series bodies, the illumination wasn't an issue, since the light was at a different angle to illuminate the AF points etched in the focus screen, compared to the points displayed on the transmissive LCD. It's not a problem in One Shot, as metering is done once. But in AI Servo, metering is done continuously, along with AF. The solution for the 1D X required that the illumination blink on and off, but I also suspect it may involve altering the way the data is read from the metering sensor, so the data from the red channel are eiher ignored or given less weight. That would be possible with the RGB metering sensor of the 1D X, the 5DIII doesn't have an RGB metering sensor.
Regarding AF point-linked spot metering for the 5DIII, while the AF systems are nearly the same as you state, the metering systems are vastly different. Here are the 61 AF points superimposed on the 5DIII's 63 zone iFCL metering grid:
The resolution of the 5DIII's metering sensor simply may not be high enough to support spot metering with the AF points, whereas the 100,000 pixel metering sensor of the 1D X can do so. Even when the 1D X's metering sensor reverts to zone metering (in very dim light or for flash exposure metering), it's divided into 252 zones - 4 times the density of the 5DIII's metering sensor.
I can't say for sure that those tecnhical limitations are absolute, but you might consider the possibility that there are technical reasons for those features being available on the 1D X but not on the 5DIII. After all, they did add f/8 AF to the 5DIII.