adding a header when filing a message into a specific mailbox

[Erik Colson]

Hi,

I happen to have a lot of mailboxes (~2000) to archive mails which works
fine with gnus.

However there is one specific mailbox I use to archive mails concerning
projects which never got executed. This mailbox holds a lot of messages
which are not really searchable.

So I was thinking how I could achieve some kind of searching in this
specific mailbox and I got an idea: why not add a specific header
(i.e. "x-tag:") when filing messages into this specific mailbox? So I'd
need gnus to ask me a question (tags) when it detects that I want to
file messages into this specific mailbox. Then gnus can add the x-tag
header to the messages before filing them. Another option is to write a
specific function for this...

What do you guys think about this?
Maybe this already exists?
Or maybe this idea is nuts?

any ideas welcome !
-- 
erik colson