I am in the throes of wrestling with this precise conundrum - complying with our prime's request to provide detailed backup documentation regarding travel expenses billed with our invoices, while protecting our proprietary indirect rate data. In some cases it seems our primes are asking for the detail just so they can do some reverse-math and determine our indirect rates, which may give them a competitive leg-up on future bids.
I will continue to follow this thread. Thanks for the insights!