The main reason is the handwriting feature, yes. Maybe it could be implemented on top of Anki, but it seems more work than to write the SRS part myself. The recognition unfortunately doesn't work flawlessly, but the native Japanese who have tried it said it works well.
I haven't done any study on if it's more reliable compared to self-reporting, though it would be very interesting to do so. From my experience, with self reporting there are some subtle mistakes you can make that you won't notice yourself so you can't correct them. For example, when writing 田 it's easy to write the middle two strokes in the wrong order. But when self reporting it's hard to notice this since when written it looks the same.
I'll take a look at the forums you suggested. Thanks!
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.
I'd love to do a real study on the effectiveness. But realistically, I don't see it happening since I don't have the resources to set up a scientific study. But if you're interested or know someone who would be, let me know. =)
One plus from the algorithmic approach is that you really have to write each time. There's no way to skip it and just look at the answer even for easy ones you think you remember. Therefore you get a lot of practice writing and hopefully writing the characters sticks like "muscle memory". Of course, if you're disciplined you can do the same with for example Anki and pen and paper, but not everyone (for example me...) are that disciplined.
That one's easy, just remember it's "opposite" - in 左, the first stroke is to the right, whereas in 右, the first stroke is to the left.
Is your SRS algorithm based on one of the published Supermemo algorithms? If so, is it one of the older ones like SM-2 or one of the complex ones that eliminates the "whoops I missed a day" problem?
Rather than make a custom deck for Anki or Mnemosyne, you're reinventing the wheel a lot by writing your own SRS. Is the main reason so you can have that awesome-sounding handwriting matching feature? Does that handwriting feature work flawlessly for native Japanese control groups? Have you found that the Supermemo/Anki/Mnemosyne approach of users self-reporting their accuracy isn't as reliable as using handwriting recognition?
As for marketing, there's a sizeable community of people already using spaced repetition software for learning Japanese, and they tend to be centered around forum.koohii.com and alljapaneseallthetime.com (although I'm not sure how much activity the latter gets nowadays). They use SRS to commit to memory the mnemonics in James Heisig's Remembering the Kanji. That community is probably the only place in the world where you can find multiple people who achieved competent Japanese literacy in a month or two. So show them what you've got and ask them how it stacks up to what they used and pick their brains for suggestions!