OK, I thought there would have been a whole lot of documenation on this, but I’ve been trying to crack this for a while (read: find something useful from somebody clever online).
I’ve tried a few implementations of PHP and XML-RPC myself, with probably the closest approach being using Keith Devens’ XML-RPC classes using this [...]

I’ve been looking for a way to edit textareas in a form with a toolbar for all the fancy formatting.
So far, the best solution appears to be Solmetra’s SPAW editor:

The page integration seems pretty straightforward. I’ll let you know how I get on.
If you are interested, there are some other options.