By popular demand (one person is popular demand here), I've added <a href="http://www.davidchart.com/Novels/Tamao/donations-in-multiple-currencies/">a page to let you donate in different currencies</a>. The default currency, for the links in the sidebar, is still yen, because that's the currency I will actually count the donations in. Unfortunately, PayPal won't let me associated multiple currencies with one button, so this appears to be the best way I can do it. Let me know if there's a currency you would like to use that isn't there; if PayPal handle it, it is pretty easy for me to add it.

I hope people find this useful.