You could possibly build the harness, but would need to buy the license from FTE to flash the ECU (can't flash without a license). There's some software/hardware in the datalink harness that communicates with the FTE software to indicate it's plugged in, I'm not sure if this is proprietary or not (probably not).

In theory you could build the harness cheap, get the software free, then just pay the $100 USD for the license. The license is locked to your FTE account and to the ECU you use it on. You can flash that ECU as much as you want, but only that ECU, unless you buy additional licenses.