Yamaha have actually implemented a variable stack length system on the R1
along the same lines as this ^^^ video. But interestingly enough, despite all
the technology they have at their disposal, they opted for a simple two length
system. The stacks are either short or long, as decided by the ECU according
the its analysis of several inputs.

I can only guess that the number of variables involved makes it impractical to
have a sliding "trombone" style stack modulated continuously like a VVTi system.
So, a pretty straightforward DIY if you are that way inclined

Cheers... jondee86