Haha 田 is actually a character whose stroke order differs between China and Japan. I'm so used to the Chinese order of the horizontal middle stroke and then the vertical one that I'd probably deliberately use the "wrong order" for Japanese. But yeah, I'd be really interested to see the effectiveness of your handwriting recognition against self-reported scores. If its innaccuracy is worse than a human's judgment of his or her performance, then it's just a gimmick that will hook people who are too lazy to learn how to use an SRS that doesn't hide its implementation details. But if it really works better than self-reporting, your app would be a real advancement to Japanese and Chinese literacy acquisition.

Also, I agree that handwriting on top of Anki would be more work than its worth, especially if you're trying to market your product as a standalone Japanese-specific program.