Would something like this work?
f = figure(...
'color',[0 1 0]);
txtbox = uicontrol(f,'Style','edit',...
'String','Enter your name here.',...
'Position',[30 50 130 20]);
Of course it's not transparency, but at least some capability to match the background color. You can of course set the color via the set option to any prefered color as well.
Maybe Yair Altman might have a better idea on it (check on the posting here .)