Hmmm... I gave a brief look at this company when I was looking for an encryption solution for USB drives (which I posted to the Coffeeshop about at the time). While I didn't crawl the entire innards of the website - I couldn't easily find any real details on how the product works. There's only this bit in their user guide:
How Folder Guard works
Though they offer a disclaimer that regular users don't need to read the details provided in this write-up just to use the software - the details themselves aren't really all that detailed. Are they dirty hooking all the filesystem API's, using a filter driver, etc? Don't know... but they seem to also be the current author of AB Commander (their own file manager), so I wonder if it works with Folder Guard? If you're a customer and ask them about supporting Opus, you may get a bit more receptivity than otherwise might be the case, since they should be able to sympathize with users of alternate file managers . On the other hand, they may look at integration with Folder Guard as an advantage they wouldn't care to extend to other competing file managers...
They also have multiple similar products (Folder Guard, USBCrypt, and My Secret Folder)... that have some overlap but which also use a variety of techniques (from simple password folder protection to actual encryption). Hopefully, the only one they'd need to provide some help to get working with Opus is Folder Guard.