One more hour of playing with Java XML RPC libraries later...
Following from my earlier discovery that the Apache XMLRPC library isn't good enough (or supported), I tried out the Redstone one. And discovered that it allows me to set _even less_ options.
So, fuck it, I guess tomorrow I'm downloading the source to the Apache one and making it work for Google App Engine. How hard can it be?
Original post on Dreamwidth - there are comments there.