I've put together this hack using autohotkey and a simple app. See it in action here:
http://screencast.com/t/k5YU5e9E
In the beginning of the video I'm creating a rough tempo map by holding down Shift+Alt+C and click on each new measure.
The second part you see me going in and adding beat-by-beat tempo adjustments and fine tuning the map.
The autohotkeys I'm using are H and J to move a marker/grid left and right respectively. It uses the loop selection to indicate distance. It's that simple!
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I've attached a zip file containing the autohotkey and other files needed to get this working on your system. I'm hopefully it works or smarter folks can develop it more - the autohotkey script sucks in particular.
It would also be my greatest wish to see a REAL solution provided someday, either with extensions or within the app itself, and it'd be especially cool to be able to click and drag the tempo markers with modifier key, or even click and drag grid lines to warp them, like is discussed in this thread:
http://forum.cockos.com/showthread.p...=tempo+stretch