I've partially solved my problem.
I'll post it here in case it's of interest to anyone else.
using 360Work's ScriptMaster plugin the very simple
does the trick. Presumably it would also work for other encodings.
I then need to bypass FileMaker's url encoding using 360's Get url as Text. However I have to correct the relative urls in this text.
If anyone has a better method of bypassing FileMaker's url encoding when setting the webviewer I'd be interested.