Probably pointless these days, but still, I'd like to upload some of my old 770 packages (and maybe some upgraded too) to Gregale extras, but I don't know how that is supposed to work.
Any pointers?

The whole process is described in the wiki: http://wiki.maemo.org/Uploading_to_Extras

The easiest way is uploading via extras assistant (web interface), if you want to upload free package.

You need to:
1. Set up maemo garage account (see Pre-requisites on wiki page for link).
2. Ask for permission to upload packages to extras (see Pre-requisites on wiki page for link, maybe I'm blind, but I couldn't find this link anywhere else).
3. Use for example extras assistant ( https://garage.maemo.org/extras-assistant/ ) to upload.

If you have questions on packaging - I had packaged debs only for Ubuntu and N900, but if you have questions or problems - I will try to help if you ask here

I wasn't asking for Chinook/diablo/fremantle, but for gregale, for the 770. Gregale is not listed in the extras assistant, but there must be some way to upload stuff for gregale extras as the download page exists, http://maemo.org/downloads/OS2006/

From the Uploading to Extras-devel page:

Yeah I see he didn't manage to read the whole page I linked :P In case it gets modified in the future or anyone else looking for this will come across this post: The section http://wiki.maemo.org/Uploading_to_Extras#Upload tells to edit file /etc/dput.cf or ~/.dput.cf adding this entry:
and then running or (if you have only one entry in /etc/dput.cf or ~/.dput.cf) simpler version:

Ah, ok... I've never used dput so didn't notice gregale was mentioned there. Thanks for pointing that out.