Sony Xperia ZR fastboot driver
While playing around with the INF files, I found that the one supplied by Sony actually does work, you only need to "force" Windows to use it for your device. I am not sure if that has anything to do with the unsigned nature of the driver or that Windows cannot verify that the driver is compatible (see below), but it simply doesn't seem to accept the driver with Sony's INF automatically.

I order to install the driver, I did the following:


In the Device Manager, right-click on the S1Boot Fastboot entry and select Update Driver Software, then Browse my computer for driver software and Let me pick from a list of device drivers on my computer.
Select Show All Devices, then Next.
Click Have Disk... and then Browse... to the android-sdk\extras\google\usb_driver directory.
Then the Model list is populated with three entries, select the one called Android Bootloader Interface and click Next.
Click Yes on the Update Driver Warning message mentioning that Windows cannot verify compatibility of the driver with the device. 
Click Install this driver software anyway on the Windows Security message saying that Windows can't verify the publisher of this driver software.


After that, I was able to run fastboot commands on my device (I only tried devices and reboot so far). 

On a side note, I found it interesting that you actually don't need any specific driver binaries such as DLLs from Sony and that the drivers supplied with the Android SDK seem to be sufficient, except for the INF entries with the correct device IDs.