Creating a custom action (no, not what you think)

I want to create a custom action, but not a scrip action, an action on a
ticket, like Comment or Reply. I have done my usual poking through the
source, and I have added some stuff to WebRT/html/Ticket/Update.html, and
lib/RT/Ticket.pm (basically copying the stuff for “Comment”. My new action
is to be called ‘Downtime’. I have gotten the Update.html to show
’Downtime’ in the dropdown ‘Update Type’ field, but when I submit, it goes
into the db table as type Comment. I am obviously missing an important
step, does anyone have any pointers? I looked in Display.html, but I’m not
grokking that one…

Thanks
Tom