Firstly, sorry for anyone waiting for updates - I don't get much time at the moment, so this is the first time in ages I've tried to do any more on this.
I've just had another go now and it looks like they've changed the system slightly at Panasonic, presumably with a recent firmware update.
What seems to happen now is that when you "slurp" home-screen.js, it downloads a poxy little thing that just want to update, rather than the full "proper" page that the TV gets. The next time you load and it serves that page, it tries to update.
I've found that if you use the same user agent as the TV uses when downloading home-screen.js (monitor useragent log in Squid) then it downloads the "proper" home-screen.js. Any other user agent and it downloads the phoney update version!
Not sure if this is a ploy to stop us amending it, or just the way it now figures out whether updates are required, but at least it's easily solved.