It looks like it is not currently possible to use Juicebox with a PHP configUrl.
I will log a bug report and hopefully this functionality will be available in a future version.
Thank you for reporting.
I got this working on my side, when sending a HTML/XML header first.
did do the trick on my side (juicbox lite version 1.0.2). Hope this helps!
BTW, really cool HTML5 gallery!