I have released rb_hijack-0.1, and it is now packaged as a rubygem to make my life easier, and to make it really simple to install and remove if you wish to

Quick summary
I have written Ruby bindings for libhijack. It isn't a complete implementation of the libhijack API, but it will provide you with the means to attach to a process and inject some shellcode to a PID that you supply from inside Ruby.
It is written as a module, so you can easily mixin it's functionality into your own module or use the module on its own.