* Draw a rectangle: width=140 pt, height=60 pt
* Fill it with a black to white gradient
* Object > Expand: Specify: 300 objects
* Ctrl+Alt+7 to release the clipping path that was created while expanding the gradient
* Brushes Palette: New Art Brush: Name the new brush *Transparent Maker*
* Art Brush options: Stroke direction from left to right
* Draw a meander stroke and apply any brush you like
* Ctrl+C/Ctrl+F to copy and paste in front
* Apply the Transparent Maker to the front brush
* If necessary modify the Transparent Maker brush weight, it must completely overlap the other brush
* Select both brush strokes
* Transparency Palette menu: Make opacity mask
* Transparency Palette: Check the Clip and Invert mask option
It could also be done with envelopes built of a gradient, an outlined stroke and the Envelope distort Make with top object command making sure that the Distort linear gradients option is checked in the envelope options dialog. But this method may be disappointing sometimes because propper gradient distorts cannot be guaranteed in all cases.
If this is for professional print, ask a prepress man if there are any doubts. Depending on how long the brush is, there might be problems.