How to add custom agenda entries?

[Marcin Borkowski]

Hi all,

I'd like to add some custom entries to my agenda.  I wrote myself
a bunch of functions to remind me when I did some things (see
https://mbork.pl/2023-06-12_Counting_time%2c_backwards), and it would be
great if I could incorporate this into the agenda.  Ideally, I would add
to my agenda setup a function which Org mode would run for every
headline in my agenda files, and display the string returned from it.

I don't think Org mode has that, but maybe there's some package which
can do it?

TIA,

-- 
Marcin Borkowski
http://mbork.pl