fMRIPrep uses tedana for optimal combination starting in v1.1.7, I believe (per poldracklab/fmriprep#1263). You could dig into the code for t2smap_workflow if you want.
On a semi-related note, there has been a lot of discussion among the tedana and fMRIPrep folks about disabling t2s_coreg (see here). You might want to avoid using --t2s-coreg when processing your data. At least for now.