Afaik there isn't much frequent work on that code, so probably few (if any) will know from the top of their head.... Meaning, everyone else has to search stuff too.
The most recent I recall (but there may be more recent) was adding the config... But not sure when/who. Best suggestion: search the git log.
I certainly haven't done any work on the coolbar code...
But, the config appears to list all the entries from the menus. Maybe also all available key commands.
So adding to those would be a first step. No idea how to add any of those as default to the coolbar.
As for adding a menu, and key command. I have done just that recently:
https://gitlab.com/freepascal.org/lazarus/lazarus/-/commit/e649b2157af99c9abb23599a4c7acfeb6545f57cIgnore the bits in fpdebug. The remainder shows where to add a new command.
Well, you can actually do that from a package too. git blame on the ide interface unit, should help you find a commit that does it.