Here’s what I’d do:
Create a null object. Basically a small shape like a square or circle. Add the Motion Path behavior to it. Change the path shape to geometry and add your paint stroke or line as the path shape. This will force the null to follow the same path as the write on to your arrow.
Then apply the track points behavior to your text bubble and use the null shape as the source. Make sure to turn off the null so it doesn’t appear in your scene.
As you may or may not know, you can limit which points in your text bubble are affected by the track points behavior. By default, they all are, but all you have to do to limit this is (with the behavior selected) CMD+click on the points you don’t want to move. They will turn blue in the canvas.
Here is a link to a sample project in case my explanation wasn’t clear enough.
423_trackpointsbehavior.motn.zip
Hope this helps.
Andy