The hidden variables returned by Classical Extras tend to be prefixed _cea or _cwp. The readme has a full list of them. “recorded at” is not currently included, but I could consider adding it in a future release (just as a hidden variable - no more UI options required). Alternatively create a simple stand-alone plugin? BUT I am not sure how well multiple plugins that all call “register_track_metadata_processor” play together, so an extra var in CE might be best.