This was very helpful! (I was just setting up an SGU for "Beyond the Bluebird" by Tommy Flanagan, and needed to create some chord triplets.)
What I have to add to this as a suggestion, is to use the percentage of tempo change instead of actual tempo, so that one can change overall song tempo without a problem.
On the Flanagan tune, I created triplet bars, compensated by +33% tempo, then to go back to normal 4/4, -25% tempo.