Adulterate summary lines

[Harry Putnam <reader@newsguy.com>]

I sometimes use gnus for certain kinds of information storage that
isn't within its real purpose.  An example might be a group called
nnml:Edited  Where I place articles I've edited with a homeboy
indexing system to be able to lookup certain kinds of info I've found
in posts on usenet.  I record what I think will be usefull lookup
strings in Keyword: header

It would be very handy to be able to make that info appear in the
summary line.  Other examples would want to insert some other info
into the summary line.

So I'm looking for a framework that I can change easily to insert
various parts of the message into the summary line.  Possible even the
matches to stipulated regex.  Or who know what all might be handy.

This needs to happen only to certain groups so must be a group-param
or agent-param induced framework.

Maybe something as simple as setting group summary line there.
I believe there is already provision in summary line formatting to
call a function and display its result.

If so, I'd like to see example code of how this might be done.
The manual describes it in some detail but I was not able to really
understand it or see how to code it:

  `u'
     User defined specifier.  The next character in the format string
     should be a letter.  Gnus will call the function
     `gnus-user-format-function-X', where X is the letter following
     `%u'.  The function will be passed the current header as argument.
     The function should return a string, which will be inserted into
     the summary just like information from any other summary
     specifier.

Where I loose it is where it says the function would be passed the
current header.  Does that mean all headers in rotation or what.
Could it be parts of the body too?

Using this option how would one make the Keywords line appear in
summary buffer or maybe the Date: line?  I see a specifier `o' that
already comes close... with:

  `o'
     The `Date' in YYYYMMDD`T'HHMMSS format.

But I'm finding that format too hard to read quickly.  I'd like to have
my own function that produces MM/DD/YY HH:MM:SS and displays it in
summary buffer.

I'm thinking it would then be possible to close group call `G p'

Change the letter following %u and call a different function.  So one
could close and reopen with a series of homemade functions if need be.

Not sure what these functions need to look like.