Many thanks to everyone for the replys - you've inspired me to download the source code for mixxx.
I really dont have a clue what i'm doing but I've got the source code so I will continue my mission to learn coding and hopefully implement the simplest solution possible for this.
Looking through the feature list, this one sounds like the most simple idea:
Combine the following two suggestions: https://bugs.launchpad.net/mixxx/+bug/674067 https://bugs.launchpad.net/mixxx/+bug/675057
And I think this would at least a start along the idea. The key is this is as simple as possible to create - the other post someone linked to with all the screen designs is great, but in my view its over engineered - at least until we recognise that this is actually a really useful feature.
My current thought is to find the way to create a button at the top of the library which will filter the list with the current playing track. Thats it, nothing else - so if you're looking in your crate - it will only select from files in your crate. And the button is just a toggle - thats all. And finally the button will only look in the "key" column - if this is empty the feature will not filter anything.
This is my idea, I will let you know if I have any success.