I had to change about two lines and now my SDWriter supports .print() and .println() in the same way Serial does, thanks to sirmorris' genius idea 
The new code is available in the same place at: http://code.jhodges.co.uk/SDWriter/SDWriter.zip
There is a new example in there called Print_Data that creates a CSV style file.
The old example is still there as writing byte values is still supported.