SASS, SCSS : pre-compiling mixins and functions? A.k.a. removing compass requirement or selectively compile mixins and functions

Question

I have a very large project made with Compass. The project takes a huge time to compile (1 to 2 mins) because it's using the Ruby version to compile (necessary because of some custom extensions that were never translated to have libsass compatibility).

I'm going to try to keep the question short, but in summary :

1. I've managed to convert most of the project to libsass
2. I've managed to go quite far into the process by using the solutions here
3. This one is supposed to add support for the image functions of compass, but generates a compiled file that is so large that it's unusable

To get out of this hell, I thought I could simply ask the SASS compiler to process only some mixins and some functions, so I can cherry-pick the compilation, make a new SCSS file that has only compiled the compass-related functions and mixins so I end up with a clean SCSS file that I can start fresh from.

Dummy example :

p{
    .alert &{
        border-color: $red;
        background-image: url('whatever.jpg');
        background-size: image-width('whatever.jpg') image-height('whatever.jpg');
    }
}

With that example, I would like to know if there's a way to compile only image-width and image-height functions or any other selected mixins. What I would like to end up with, coming from the above example :

p{
    .alert &{
        border-color: $red;
        background-image: url('whatever.jpg');
        background-size: 128px 248px;
    }
}

That way, I could clean up my SCSS with real static values, remove computation and the Compass requirement without messing with the file structures and variables.

Thanks a lot!