I didn't find a margin hack for my Sonys (505 and 300), but it is possible to edit the style sheet for epubs (though you will need to remove any drm first). However, it is also possible to ruin the look of the book if you go too gung-ho in doing this (mea culpa).
On the K3 the "hack" TheKindleWorm has pointed you to does seem to work very nicely - certainly it has held for the first three books I have read on my new K3, though I did find changing the number of words per line in Settings and then switching back to the Default words per line setting also resulted in my losing my margin change in the pref file.
I still have a very sweet spot for the styling of the Sony readers, but I already know that I will be keeping my K3 and it will now be my daily e-reader.
(Off topic slightly, but today I was looking at my progress bar and realised that it has little markers within it showing where each chapter starts - lovely).