I cannot find the discussion, but the info is here: http://www.rtl-sdr.com/tag/rpitx/
Any links to write-ups/HOWTO's would be tremendously appreciated!
This is a pretty good write up of using the Pi as a wspr transmitter: https://github.com/JamesP6000/WsprryPi
Technically it works, but it's very noisy and the transmission range isn't far enough for anything too serious.
A full duplex SDR would be a better investment.
On the range, I did not pay much attention (but maybe I should have): the power of HackRF one is low, too; one needs an amplifier to transmit in the open air anyway. Do you think that the rpitx TX power is not suitable for table-top sized prototypes?
It says a dynamic range of only 13 dB on the levels.