I came here to suggest the same thing! There's not much that was easier about the old days when I used a cassette four-track to record, but this is one of them!
I read recently that the Beatles used this trick for some of the piano parts on Abbey Road... they would adjust the tape speed slower to record a difficult part.
For the time being I guess we can ...
- Export a stereo mix of the song
- Import this audio file into a new Studio One project
- Time stretch this to be longer (ie, slower)
- Record new parts while listening to this
- Time stretch the new recording to be shorter (ie, faster, the original tempo)
- Export this new track
- Import it back into the original Studio One project
But I've gotta say ... Varispeed within Studio One would be a lot easier!