Is it possible to redirect files in a readonly filesystem?

Let's assume that I have created a linux image with a filesystem that is inherently readonly (like SquashFS) and disabled swap. From readonly I mean the main filesystem and all its content is readonly and it is mounted readonly as well, and only directories that need to be writable such as /tmp is redirected to another writable storage/partition. Now my question is that if someone has a full root access on this system, is it possible that he somehow spoof accessing files in readonly part or not? For example, if a.so is in readonly part, is it possible that all access to this file is redirected to another file? He can copy any file on the writable section of system or run them with root permission. Best Regards