An update. Calibre development has come along, and I did get a successful conversion of the Pocket OED from deDRMed mobi to epub. (Or rather, someone with a more powerful computer than mine did it for me.) The epub OED displays fine on my PRS700. However... (you knew there was going to be a however, didn't you?)...it's only marginally useful, because the built-in search links don't work, and the Reader's Search function takes so long you can have lunch while you're waiting.
When I use the Reader's Go to Page function and the scroll bar to advance through the book, I can get to a word I want in less time--though it would still probably be faster to go get the paper dictionary off the shelf. Or grab my PDA and use the original mobipocket version on it.
I am presently attempting a conversion from the epub to lrf to see if that format works better. But my computer's been cranking for 15 hours on that and it hasn't finished yet.
I was curious whether response time was faster from the Reader's memory or from the SD card, so I put it on both. I think it's a little faster from main memory, but I didn't do a careful study.