I don't know that this will help but one of the attachment examples in the IBM WS developer pack uses RPC and dynamic invocation to send 3 files ( 2 of them binary ) as RPC attachments.
One of the files in the sample is 3K. IBM WSTK download at: http://www.alphaworks.ibm.com/webservices after you install the WSTK, the sample RPC attachment is located at %install_root%\services\demos\attachments
