Hello,

I decided to implement the custom transform idea I posted earlier and send a pull request: https://github.com/imageworks/OpenColorIO/pull/390

The implementation is quite short and simple, it just exposes a class that new Transforms can inherit. By implementing all the functions, it can then be used just like regular transforms.
This is particularly useful for customizing a OCIO-based color pipeline, since own additions might be needed.
In my case, this functionality is required for implementing a tonemapping operator in Blender, which uses OCIO for its color management. Currently, the only two options are to either maintain a modified version of OCIO, or to implement it alongside OCIO before or after the Processor is called. With this change, it would be possible to implement the tonemapper as a Transform and directly use it in the DisplayTransform.

An example that shows how a custom transform looks like are these two files (header and source):
http://www.pasteall.org/56658
http://www.pasteall.org/56657

So, I hope that this feature will be accepted, and will of course fix any issues that might be present.

Best regards,
Lukas Stockner