I don't recall there is any straight-forward way to do it from plain ReaScript.
You could suggest to Julian Sader, the developer of the js_ReaScriptAPI extension plugin, to add something for that. I might also myself look if there's some sensible way to add something into that plugin to allow that. Reaper does have a mechanism available for C++ extension plugins to play a PCM_source object (read, "audio file") into the main audio hardware output.
edit : I already got the playback working. But as usual, stopping (properly) is more complicated than anticipated...