There are two examples in the projpred source code, that use init_refmodel() to define reference models for rstanarm and brms models (see for example https://github.com/stan-dev/projpred/blob/181606a416471f8145976e8b048d5213b9fb6ab3/R/refmodel.R#L116). By following those functions, you’ll see how to set up the various parameters expected by init_refmodel().
Having said that, I agree that documentation could be improved on that front! :)