Been trying to make this happen, but no luck so far. I want to get Hydrogen to change tempo in the middle of a song. Since I read that it is not a feature available yet, it seems there is a workaround by setting that tempo change in Ardour, then slaving Hydrogen so that it captures the change and reproduces the drums accordingly.
In order to make this happen, I have set up Ardour time signature changes to match Hydrogen’s, so that it’s all the same bars and beats throughout for both of them. Then I clicked on Ardour “Time Master” button. I then have tried to find a way to set Hydrogen as a slave, but have found nothing.
In summary, Hydrogen ignores the tempo change and continues to use its own tempo.
For those interested, it is indeed possible by using the latest versions of Hydrogen (I have used Beta 3). Having said so, keep in mind that it only works if Ardour and Hydrogen are EXACTLY the same in terms of beats and bars. Also be aware that these betas have other issues as well…