@Ehsan the suggestion worked, so once again thanks! The tk-multi-setframerange app is defined in the project environment, and gets called from there, with context explicitly passed from scene_operation.
The Houdini engine is a bit puzzling, there is a timer that scans the file name and changes context when it detects a new one. Perhaps someone who worked on the implementation could shed some light? Not critical at this point, but would be interesting and potentially useful for further customizations.