I've found the Genuine Fractals Print Pro 4.1 plugin that (besides high quality upscaling) adds the capability to open/save .stn files in old Photoshops (7). I'm not 100% sure about its origin but VirusTotal.com finds it clean. It's a trial version and can't recall if there are some limitations (watermarks etc.).
It's for Windows and it's less than 5MB. If you are interested, send me PM and I'll put it somewhere to download.