Sadly this has had no proper answer for over 3 years. LimeSDR would have such a potential for all of us that want to use it on HF for experiments with gnuradio.
Allegedly someone manged to get it it working long enough to make some actual HF contacts using this software called sdrangel LimeSDR USB Version Transmit on HF - Here's the verdict (and it's GOOD!) - #8 by kiromn
No idea why it doesn’t work using gnuradio and does work with sdrangel. Pity this fine hardware suffers such poor community support for HF.