Well the picture in my first post (
#7) was just a mockup I quickly wrote using the Qt framework (C++), and not running inside of REAPER.
The picture in #22 is the "real thing". There is a long way from a simple Hello World dialog to a working prototype (and I'm new to WDL and the Win32/Win32-like API so that's extra work).
EDIT: Got the file downloading code working.