Well I found "build.prop Editor" worked fine - available free on the Playstore
On the Fire tab installed "Bluetooth GPS" and tested on two older Samsung android phones (GT5670 and i5500) a program called "GPS over BT" both free and available on Playstore.
Working fine just need the build prop edit as per OP and enable mock GPS provider.
Google Play don't let me install "GPS over BT" on my Fire.
Which firmware are you using ?