I think you will need some RAM disk utility to which you can in runtime create your decrypted DLL, then load it from there.
If this is considered by your requirements as not secure, I think you will have to implement such RAM disk functionality by your self, which you then can contain in your executable.
But would explore throughly the options of available RAM disk utilities/libs, or similar solutions.

Another way would be to take the original code for QLibrary (on which QPluginLoader is based) and and change it to take your binary content.

Not trivial.