I've been using ClickOnce to distribute apps within our corporate network for quite a while and I do not think the default install directory can be modified.
If your application does not require any files other than those that reside in the bin\release directory then try copying the necessary files directly to the flash drive (bypassing the deployment altogether). Depending on the complexity of your application it may work just fine. If your ClickOnce includes the .NET Framework and other required components than this option most likely will not work.
Best Regards,
Earl Francis