In fanfiction, the problem is solved (if the writer cares) collaboratively-- American writers trying to do British English well is such a common problem that the proof-reading and copy-editing has a name: Britpicking. I assume that most of that is done by native speakers.
The problems can be subtle. I was shocked to find that modern British English doesn't include "gotten". How do they make it through the day without such a useful word?
And I'm not going to mention the book because the author's a friend, but she writes excellent British English. When she had a couple of short passages of American dialogue, the result was agonizing. She didn't make the typical error of exaggerating differences, but there was something very wrong with the rhythm.
Aside: As an American, I've often been quite surprised to find out that authors were British (if I read the books before I got background on the author.) My reaction is "British? It can't be!" This has happened with Alan Moore, Douglas Adams, Neil Gaiman, and Charles Stross. I wonder when my brain will figure out that not everyone who's fun to read is from my home country.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.