How can a datapump export have its resource usage limited on windows?
Send it over the network

While extensive searching found no way to limit disk io natively (and serverFault will probably have more details on that) a "natural" way of limiting IO is to send the output directly across the network without ever touching the local disk. This then allows you to take advantage of all the network IO throttling resources out there. (If you want to be really sneaky, try to set up the network resource on the local machine, but that may be an overly-cute solution.)