I couldn't try this while in Greece but I'm now back in the UK trying to set the N810 for my next trip. I installed privoxy 3.0.6 and put this in user.action (at the end of the file):
This follows the example in the manual here and seems to be recommended rather than content-type-overwrite.

Result: the privoxy daemon fails. I haven't found an error message anywhere yet.

Does this work for anyone else?

Also, I tried setting 'enable-edit-actions' in config but it didn't. Does the web-based editor work in this version?

Dave