I got something working in the end, so in case its ever helpful to someone else, I followed the idea suggested in
http://www.rawmaterialsoftware.com/viewtopic.php?f=8&t=6901&p=44191&hilit=texteditor#p44191
of getting my text edit boxes to create a separate modal window on the desktop to accept the input when they get focus, and then close down the modal window when they loose focus.
Seems to work for me in Windows and OSX (although I need to run a full compatibility test on it in different DAWs, etc).
Hope that helps someone.
Paul