There is a way (although it is quite indirect) to create a tempo map in a Dorico project today.
If you have a sequencer that can create a tempo map either by tap tempo or by beat mapping, then do the following:
- generate that tempo map in the sequencer
- create a midi track that tracks the tempo map
- export that track as a midi file
- import the midi file into Sibelius
- export that file as musicXML
- import that into Dorico.
Steps 4 and 5 could be avoided if/once Dorico can import tempo from a MIDI file.
This seems like a lot of work, but it is not really all that difficult.
For an example of creating a beat map using Logic Pro X from an audio file, see this video tutorial. One could do something similar using tap tempo if an audio file is not available for the project.
The video goes as far as creating a drum map synchronized to the tempo map. You then need to convert that drum track to a midi track and then do the exports and imports as described above.
The above may or may not work for some interim sequencers and/or notation programs. But it does work using Logic Pro X and Sibelius.