I was quoting from memory and obviously got it wrong.

I've got some pages copied from a Japanese book about the origins of their characters. Kanji came first, probably around AD400 as you indicated (that would be where I got "400" from :-). Both Hiragana and Katakana were derived from a Kanji - I've got a chart showing the original characters* - but for different reasons and by different people. From the literature I have, the idea of reserving Katakana for foreign loanwords and onomatapeia is not mentioned. I suspect Katakana was used for particles and word endings before the male scribes got a hold of Hiragana - that's what a Japanese language teacher told me Katakana was devised for anyway - but knowing the Japanese aesthetic, someone probably decided Hiragana would look better doing that job.

Wade.

* When I find it, I'll scan it and post it up.