Can I change instance icon depending on their "Tag"?

I wanna make pseudo classes that I can use and it would help out if I could somehow assign a special icon for those. Is there any way to manipulate the icon that’s displayed on an instance. I couldn’t find an API for it so I assume it’s not possible.

In the Explorer? If so, then no - you can’t change the icons dynamically. They are going to be the same for every occurance of the Instance.