Unfortunately I can't speak to GuyEngine's functionality alongside any 3rd party apps, plugins, or chaining tools. You're right that VST3 isn't supported in Pro Tools, but VST2 is (if I'm not mistaken) and there is a VST2 version of GuyEngine. You may have better luck with that. It's not a very resource intensive plugin so it should still perform fine. The VST2 version is functionally identical and draws content from the same files. Would you be willing to give that a try instead?
I'm unfortunately having the exact same problem with the vst2 version. I've extracted the GuyEngine 1.0.4 VST2 .zip folder into the C:\Program Files\VSTPlugins folder which didn't work. I've also tried the C:\Program Files\Common Files\VST2\ folder which also didn't work. I should also note that there are functional plugins that exist in both of these folders, so I just really have no idea why GuyEngine won't appear in my library. If you have any other ideas on what I can try, please let me know. I really would love to use this plugin!! Thanks.