Joachim,
I tried the <Alt-down> for a little bit and I do like it. The only thing I notice is that once the Alt key is pressed, there is some visual feedback in the window menubar, in terms of underlining the first character in the menu items. However, it's so subtle that I don't believe the user would even notice this unless they were looking for it.
The only other issue I can think of, which is nit-picky, is if halfway through the hotkey sequence the user decided not to invoke code assist, but because they already pressed the <Alt> key, the focus is now on the first item in the menubar. This is in contrast to <Ctrl-Space> where pressing the <Ctrl> key, by itself, does nothing. However, this would also happen in Excel and I'm not clear on how often this usecase would occur.
I think this is worthwhile, maybe I'll make sure that this secondary trigger can be turned off easily.
Seth