In order to properly send tempo changes to external synths, it would be a good idea to introduce a new macro variable "t" for tempo changes, since there is no standardized way to tell the tempo via MIDI. However, I have no idea how various synths / workstations handle tempo changes and if the tempo always uses the same format (you'd need at least two times 7 bit to transfer a BPM value, but everyone could do that in different ways), so it's probably not easily doable.