It seems to me that, unless an author sets out to script just 
another mediocre Nettouhen episode, he or she must do 
something with the characters that had not been done by 
Takahashi or the anime writers.  Keep in mind that the original 
Ranma 1/2 characters were:

1. One-dimensional (except for Ranma, Akane, and maybe Ryouga)
2. Poorly developed (even Ranma and Akane are almost completely 
static after the Hiryu Shoutenha story)
3. 100 percent predictable (except for Ryouga, and he was only 
unreadable in situations where his innate nobility conflicted 
with his desire for vengeance against Ranma)

So, if and when authors try to break these bounds, are they 
guilty of that too-prevalent critique, "OOC" writing?

I feel that if the writer of a fic is attempting a serious storyline within 
the Ranmaverse, changing the characters can be nessecary for reasons well 
stated in the above list.  'A Lesson in Love' was an excellent example of 
changing the characters in order to write a brilliant fic.  (Which I shall be 
reviewing soon, to give fair warning).  You will notice when I do this review 
that Marisa's Characterization rating will not suffer dearly because of the 
way the changes are brought about and the purpose they serve.  

The author is not writing out of character, he or she is 'adding to the 
character'.  If they remained static throughout fics I would stop reading 
them.