Thankfully, Nencho provided a workaround, JavaScript:

function AppLoaded() {
    var async = $find("<%= Me.RadAsyncUpload1.ClientID%>");
    async.set_dropZones("#dropZone1,.otherDropZones");
    async._uploadModule._handleCustomDropZones();
}
Sys.Application.add_load(AppLoaded);

Having the ability to set custom drop zones will make it much easier for advanced scenarios where, for example, the RadAsyncUpload control is extended and dependent upon other client-side controls/client IDs, especially when the majority of the extension is client-side functionality wrapped within a custom user control.

Otherwise, the following forum post by Hristo is invalid:
http://www.telerik.com/forums/asyncupload-drop-zone-define-in-javascript#VilSKc_gOUGmhJhzfbr8uQ