I've got the Color Unpremultiply blues...

I have a problem with some images I’ve created, initially I didn’t know what was happening but after some digging around it appears I have output these images without enabling the ‘Color Unpremultiply’ option in the Scene tab. This is a real problem as I’ve already supplied production files to my client and while realising the transparency wasn’t quite right I thought it could be fixed in post production at their end.

The images are only stills but make extensive use of motion blur so there is a lot of transparency. The target or correct image is image 1 below. I ended up supplying the files in EXR format (image 4) as this was the only way to preserve the correct transparency effect. However when converted to an 8 Bit format like PNG I get an effect similar to image 2, which is not acceptable. These files were output using Blender 2.65, Blender internal.

Does anyone know if it is possible to output these images in Blender, using Compositor nodes perhaps, and still maintain the ‘Color Unpremultiply – off’ effect, but with alpha? Image 1 or 4 is what I need to reproduce. If not, is it possible to do something in Photoshop when converting from a 32 Bit EXR to 8 Bit TIFF?

I am a relative newcomer to Blender so have only limited knowledge when it comes to more advanced areas like the compositor, so please point me to a tutorial if you know any that may be appropriate.

Thanks in advance.