Does Toei Eigamura offer discounts on the admission price for dressing up in a kimono?
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 wafuku (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 wafuku. 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.