Just thought I would report back, in case anyone is interested.
I could not find out how to set a batch target filesize in fireworks. (I know you can on individual export, and that you may be able to go to history, and save that as a command to use in a batch - but seems a bit fiddly).
I downloaded
http://www.irfanview.com/english.htm . This is a great program, does lots of jpg batch processing stuff, but could not see how to use a target filesize.
I then tried
http://www.thumbnow.com/software/#ThumbNow . It did exactly what I wanted. Dead Easy. Every file is now exactly 50 Kb. That is just about all it does, but it does it really well.