I tried using a cygwin tail, but it locks the file so the original process can't write to it.

LOL -- that's the best joke I've seen about MS Windows in a while. I suppose it could just be taken as a (yet another?) case of midguided implementation on the part of cygwin, but based on the whole brain-damaged approach to file locking foisted by MS, cygwin should not take all the blame (or should at least admit only to justifiable failure).