Steve, my USB interface works this way. Altough I stopped development some time ago, it is already working. Some people had built it and reviewed it positively.
It uses a DC-DC converter to generate 9.6V using the USB 5V, and powers the Futaba TX via the trainer connector.
See the original thread here [url=http://n.ethz.ch/student/mmoeller/fms/forum/viewtopic.php?t=3224]New USB RC interface project - USB interface with PIC16C745
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If desired, I can send the files via e-mail.