Support for fieldmap-less (SyN-based) susceptibility distortion correction

I'm trying to run SDC on datasets without fieldmaps, using SyN-based estimation (as SDCFlow/fMRIPrep support via --use-syn-sdc). I've tried using --bold_sdc TRUE without fieldmaps, which causes the bold_fieldmap process to fail rather than falling back to fieldmap-less estimation.  Is there support or plan to support fieldmap-less SDC? If not exposed yet, is there a recommended workaround?