A suggestion for a simple drop down list/ menu per track which contains all of the instruments present in that track (whole track in song). Selecting an instrument from the list simply selects it in the instrument list
When the show menu is closed, all that you would see is the hilighted in red drop down, or similar icon. This could be placed more neatly than is shown in the mock-up.
5787 
Not a bad idea.
Personally though, I’d rather see a some type of automatic sorting function in the sample-list just like how the patterns are being numerically (all the samples connected to track 1 will be at the top etc). At least for me, it would made more sense upon browsing the tracks.
In the upcoming version, I read somewhere/if I’m not entirely incorrect, there will be a feature to group & color the samples. Additionally, I had the idea that these sample-colors could be automatically linked to the track-color (https://forum.renoise.com/t/multisample-handling-and-instrument-text-display/43791).
Moreover, it would made more sense (at least to me), upon browsing the tracks, the samples which are used there would be highlighted in the sample-list. But sure, it could be combined with your idea.
There could be even smarter solutions than this, but it would perhaps be better than the current system imo.
Not a bad idea.
Personally though, I’d rather see a some type of automatic sorting function in the sample-list just like how the patterns are being numerically (all the samples connected to track 1 will be at the top etc). At least for me, it would made more sense upon browsing the tracks.
Joule has written quite a handy tool that does this from the right-click menu. The relevant sub menu option is Apperance (From Track). It is not automatic in the sense you have to initialise it though.
https://forum.renoise.com/t/updated-tool-3-1-sort-instruments/34208
In the upcoming version, I read somewhere/if I’m not entirely incorrect, there will be a feature to group & color the samples. Additionally, I had the idea that these sample-colors could be automatically linked to the track-color (https://forum.renoise.com/t/multisample-handling-and-instrument-text-display/43791).
Moreover, it would made more sense (at least to me), upon browsing the tracks, the samples which are used there would be highlighted in the sample-list. But sure, it could be combined with your idea.
There could be even smarter solutions than this, but it would perhaps be better than the current system imo.
Yes if if something with the track colours could be added to the instrument list it might be handy too. Maybe the orange squares that act as trigger notifiers could be coloured. Hilighting while track is selected would be handy too.
The reason I would still be in favour of a track drop-down / menu though is:
It saves scanning to the instrument list so you can keep your eyes on the track you are working on.
It gives you a method of fast selection alternative to autocapture
I can see some scripting side benefits 
The thing that got me to thinking about this again was I had to implement a sort of traffic-lights system in my latest script to manage all this. Even then there are some ambiguous situations.
The reason I would still be in favour of a track drop-down / menu though is:
It saves scanning to the instrument list so you can keep your eyes on the track you are working on.
It gives you a method of fast selection alternative to autocapture
I can see some scripting side benefits
also: