This is possible: you can setup a threshold in Main Menu > Training Settings > Interval Training. If you define just one interval, this interval is always active. At the same time you can set the frequency of announcements to 1 hour so you won't get the regular messages.
Just installed 2.3.0 and I noticed that in the Voice options screen, you can select heart rate to be spoken (even though the current manual sec 4.7.1 does not include heart rate in the list) at intervals. While this is nice, I would prefer to have heart rate spoken ONLY if a certain threshold is met or exceeded. Can this be added as an option/condition?
For example, I could set heart rate threshold at 160bpm and when I exceed this value during a training, it gives me an audible alarm or voice prompt.