[support] Extra sitewide variables/configuration - where best to put them?

Hi, We're building a site for a client that's taking donations on a rolling basis. We'd therefore like to have e.g. the running total of donations stored in some centralized place that the client can update, and I was wondering where would be the right place to put that. Any ideas? Extra input fields on admin/settings/site-information would feel right, but I'm not sure how to hook into that or if so how to get at the additional content from within PHPTemplate. Best regards, J-P