Thank you for pointing this issue, indeed RadAsyncUpload will process the files on a postback, no matter whether it is from the submit button or from other control. Unfortunately, a workaround is not supported out of the box. However, the one that you have found is fine one. We will brainstorm on how to handle such scenarios and probably implement an out of the box workaround for service pack 2. Until then, I suggest that you stick to your current approach.
the Telerik team