what sort of information?

As I said, the information to be displayed could be about the sender of the currently highlighted email in the message list, or some relevant Python-generated statistics about the previous emails from that contact, or some related database-retrieved info, or any other context-rellevant information that could be useful (as specified by the user); this could be used also when composing emails. There is an endless range of possibilities here. All of this would be managed by the Python scripts. The only thing that we need from Claws Mail is the possibility of displaying some info within a frame.
This enhancement could be seen as a sort of virtual assistant or secretary: while you are doing your own thing within the email client, there is a 'secretary' that reminds you of this or that relevant fact, based on the context of what you are doing. By pushing this concept even further, you could even actively interact with the 'secretary' by requesting a more detailed info about something, but this is something totally outside the scope of the feature that I am requesting.

Bottomline: all I am asking for is a content frame that can be used by Python scripts for a variety of uses.

You can probably already achieve what you want, (depending on precisely what it is you want), using the Actions feature, scripting, and kdialog or zenity or similar.

Not quite. The lack of a frame integrated within the UI is quite a limitation. 
I am afraid that external programs such as kdialog or zenity, which you suggested, are not good enough for this purpose - the rendering would be ugly and inconvenient, with overlapping windows which will cover part of the screen.

As for scripting, what I have in mind is a Python script that constantly monitors CM's events, and visually outputs some data in real time. For that we need an output pane.