I'm not familiar with the internals, but I'm just trying to figure out what you're asking for. It doesn't seem like it has any relation to JDT. What do you mean by "where I'm actually editing"? Obviously, that is the currently focused editor tab, but you must be referring to something else.

It seems to me though that what you're describing as a solution is much more general than just JDT. I.e., it seems to me that it would indeed be useful to have a general UI capability to mark any editor tab (one or more?) as "favorite" such that it is visibly decorated in a somewhat eye-catching way so that one can easily find it visually. But rather than implementing that as a separate plugin, where you'll always have the problem of digging deep in the internals, instead contributing such a thing directly to the Platform UI project so it's generally available everywhere (via the context menu on the editor tabs).