score:2
As far as I can tell, the answer is no.
I rocked up to the ticket gate wearing a yukata, obi and geta, asked in Japanese if there was a waf*ku (Japanese clothing) discount, while accompanied by a native speaker of Japanese, and I was told there wasn't. Also, there was no mention of such a discount on the English-language edition of the mobile friendly version of their official web site. In addition, very few Japanese visitors to the park were wearing waf*ku. It is possible that the staff member got it wrong, or that I had to be wearing a full-blown kimono rather than a yukata, but I doubt it. The most plausible explanation is that such a discount existed, but doesn't any longer.
The men's toilets within the park are sufficient for changing from normal clothing into at least a yukata, or doing the reverse.