I think the question should be: what do you want to achieve? What do you want functionally? Or are you going to implement something for the sake of it?
1. I want to learn how to use HMVC 2. I want to reuse some of my routines in future websites.
I have used HMVC in Codeigniter (with addon) and just had to create a modules folder and add folders for controllers, models and views. Then just use modules::run
oil did a nice job for my admin back-end, however I needed to write a forget password routine. I was thinking it would be easier to drop this in in future apps. Or can oil be modified to easily add this?
They are identical in structure to your app folder, but all classes in it have a namespace equal to the module name.
So say you have a controller named "Controller_Login" in your app, and you move it to a module called "admin", it will become "\Admin\Controller_Login" (so "Controller_Login" in the namespace "Admin"). Likewise, the URI will change from "\login\action" to "\admin\login\action".
So all those special things and strange constructions you had to use in CI, forget about it.