Changing a field title in a panel I've got a field in a panel. I want the field title to be replaced by something that is dependent on another node. In a view this is really straightforward: 

View -> Configure Field -> Rewrite Results -> Rewrite the output of this field: ([field_A] is the name of this thing)

The Panel, however, doesn't recognize the [bracket] functionality in Node -> Settings -> Override title.

Apologies if this is obvious, but I can't find the solution anywhere. Any help would be hugely appreciated.
You can use keywords from contexts when you override a title of a pane in Panels. Go to Contexts in the Page Manager UI. The summary of contexts displays the available keywords. 

In your situation you may want to add a new context, the node that you want your pane title to be pre-populated from. You can select the node ID, and the same node will always be loaded.

If it's not always the same node that you want to use, but e.g. a reference field in your already loaded node defines it, you can use a relationship to create the appropriate node context.