Oh, Gouache?? I never even heard of someone trying it this way! Boy, ya' learn something new everyday.
Let me ask you, did the gouache completely remove itself after you washed it?? Aren't you worried about the water getting underneath the ink and lifting it?? I am assuming you are using waterproof ink, but I would worry about the absorption of the paper.
I'll admit, you probably saved a good brush, that frisket sure does have a way of ruining one!
Really cool! Thanks for sharing!!