I have had this same problem before too. Here was my workaround (not nearly as productive and time-saving as Mr. LaRonde’s, but academically, a little more challenging).
First, I had to duplicate the write-on effect to use as a source.
Then, in the original write on, I wrote an expression that used the position of the brush from the duplicated effect and then adjusted the x,y coordinate of the brush position until it matched that of my mask. A little trial and error, but ultimately, it worked like a charm.
HOWEVER, now I know how to make it work WITHOUT spending an extra 20 minutes setting things up.
Live and learn, right??
Jacob