Given that your in-browser demo and your reply about having changed what the demo contains make it clear you’re not restricted by “This is a custom engine written directly against Win32 APIs”, would you please consider offering a non-Windows option on GOG.com?
If you’re not willing to provide native Linux and macOS builds, is there any chance you’d at least be willing to talk to GOG.com about offering a non-demo browser build in a zip file as an extra?
I can report that the demo plays perfectly well in Ungoogled Chromium on Linux.