The OpenMAX|IL core provides some basic functionality (much by provided macros). A simple implementation doesn’t take much code.
Section 3.2 of the IL 1.1.2 specificaiton covers the Core.
A basic core doesn’t have many OS dependencies. Most of the complexity in implementing an IL Core is from optional support, e.g. dynamic discovery of components. I don’t know of any non-basic implementations that are public.
I don’t believe that one is currently included in the Adopters’ package (other than the macros in the header file).
There is an Open Source implementation of OpenMAX IL called Bellagio (http://omxil.sourceforge.net/index.html). This includes a basic IL core, which, licensing issues aside, may be a good place to start. I would caution that this implementation is not conformant, but I would not expect significant issues with its Core implementation.