Surely there must be a mod or modder who can make a mod that locks W to always down on command. Anyone?
Idea: mod makes it so that hitting ctrl+shift+w locks it to down, and tapping w again releases the lock.
Here you go: http://spacedock.info/mod/962/Wwwwwwwww
Written for the 1.2 pre-release but probably works on earlier versions as well. Doesn't actually use much of anything from KSP, just Unity and Windows. Probably doesn't work on other operating systems. MIT license so do whatever you want with it.