Failing that, can anyone advise me on which of these libraries have the most haskell newbie (LYAH trained, but still struggling with RWH) friendly approach to writing new conversion functions?
You may get some responses here, and you'll find that if you ask on cafe, there are many more people with practical experience of the sort you're asking.
This list was forked from cafe because there were complaints that some of the questions being asked on cafe were too simple, i.e. at the level of LYAH, e.g. monads, number type class, etc.
Over time, haskell-beginners has gotten really good at such questions. YMMV for anything else.