There are 2 major problems with this.
A. This system must be very precise, tough, cheap, and relatively light. Nobody has combined all those things.
1. Especially in high speed/suspension stress situations, you have many variables to contend with. Suspension squirm (bushings, mounts, etc) that vary from year to year and road to road, grip levels, road conditions (bumpy, wet, snow, etc), so on and so forth. To get a system to preform consistently in the same way in almost all conditions is very very hard. You certainly don't want your rear toe to change mid corner when you hit a damp patch, bump, or when your tires get hot and start to lose grip.
2. Although the rear toe isn't changing on a car, especially in high-performance or racing driving, it's easier and better to adjust your driving style to the condition rather than introduce a new and possibly inconsistant variable.
C. Say you do all these things. Is it going to fit on the car and be cost-effective? If it worked great, but cost $7,000 to put on a Focus, I don't think you'd have many takers.
It may come someday, but like active suspensions on high-end cars today, it's got to fit the application on use and price.
Don't waste another minute waiting on parts, or anything else. Just get out there and start racing! You will suck at first, and you will be slower than many other cars, but the only thing you will regret is not starting earlier...