Malcolm,
There's a concern that if you only write, say, 2 of the last 8 bits, this won't actually get written unless you call flushByte. I don't know how this was handled in the NHC version; perhaps just require the user to call flushByte at the end if they're worried about such behavior?
The final-byte flush should be automatic when the BinHandle is closed.
The GHC version lacks a close function. Presumably this is the 'closeBin' function which is commented out in the export list. All this would need to do is call flushByte, assuming we don't allow AppendMode? Though if we force them to call flushByte at the end, instead of some otherly-named function, presumably they could use AppendMode as long as they realize that they need to flushBytes at the end of any write. I don't particularly care...I don't find appending binary files to be very useful.