Kinda late to the party at this time but bear in mind: unload can blindly unload to a fifo.

So if you create a fifo and have a program reading from the fifo and appending to your target file, you can throw several UNLOAD outputs into the fifo and have the data from all those unloaded queries sent to the one file, courtesy of the fifo-reader.

I did this years ago, unloading to a FIFO and having a dbimport (dbload? I forget) reading the FIFO to load another table. Very fast table copy!