Display something besides Subject line on request

[Harry Putnam <reader@newsguy.com>]

I've successfully confused myself beyond all reason while reading the
gnus manual again...

First the wanted functionality:

To be able to cause the display of a different header in summary lines
on request.  Or on group parameter basis.

I think this is different than the To From stuff delt with at:

(How do those cool urls work again?)
info://gnus/To_From_Newsgroup     <== naa

C-h i m gnus g To From Newsgroups <RET>

I don't want to call this on the basis of From or other header.

Imagine this proceedure:

Assume I sometimes edit messages and add my own Keywords: header.
Later, to find messages based on info in that header I might say 
`& Keywords <RET> REGEXP <RET>' and at the command prompt: `#'. 

Gnus will dutifully tear thru the summary buffer marking any messages
with a keywords header containing REGEXP with `#'.  I can then view
those messages with `/ n'

At this point I want that limited buffer to display the Keywords
header instead of the Subject header.
When I pop that limit I want the Subject back.  Either by
M-x <something> or by hooking into the limiting some how.
I guess M-x <something> is the more reasonable.
One could say M-x <something> to  toggle Keywords and M-x <something>
again to go back to Subject.

This is different than making gnus display the Keywords header in the
visible headers when a message is opened.  I know how to do that.

This snippet from the above cited info section seems to be close to
what I'm looking for:

  2. The value of these extra headers can be accessed via the
     `gnus-extra-header' function.  Here's a format line spec that will
     access the `X-Newsreader' header:

          "%~(form (gnus-extra-header 'X-Newsreader))@"

But it doesn't really indicate where this goes.  The summary format
strings given as examples contain nothing like this.  Nor is it clear
if the `access' can be done at will or if it is a permanent part of
format string or what.

Further the Keywords line I may have added by gnus-edit-article will
not be in the nov file.  Unless I've run nnml-generate-nov... after
adding it and have code in .gnus to include keywords in the nov file.
This I also know how to do, but its not really what is necessary to get
the results described above.  Or at least not a very practicle way. 

Can something like this be done short of 8127 lines of complex lisp
code? And wearing out two keyboard getting the right lines?