DONE (REAPER v5.983)
(from
here)
We can load a project template via Main_openProject() (as a project template is 'just' an .rpp) but it's different to doing it via 'File -> Open project template', because this way Reaper remains in a 'clean' state (as in no 'normal' / non-template project opened), e.g. the default 'Save as..." path is not changed.