This particular change, in it's complete form, would be challenging in my opinion. Many other kinds of planner changes aren't so bad though, but people (including me) tend to avoid them.

Executor changes are much less intimidating and lots of people make executor changes. In my opinion, the executor is the most approachable/easiest major component.

I would encourage people to not get intimidated though. For every project where I was determined enough, I succeeded. Tom Lane does especially amazing things, of course, but I think reasoning like "I'm not Tom Lane, therefore the planner is too hard" is the wrong lesson to take away.

For reference, a college-level CS databases course I took had us hack on the Postgres query planner to improve a very specific case such as this.

It's large and slightly complicated, but damn it's clean and understandable. Or at least it was back in 2005...