Ok, I just installed fluxbox1.0rc1 and the need for Mod0 is gone, you can just put in F5 by itself, for example, and that's all you need to press. If you install this don't bother using the .fluxbox folder from the previous one. It won't start up, or didn't for me. I just copied menu keys and startup but nothing else. Not sure what the issue was but probably difference in init.
Oh btw, in this version, that nasty maximization gap on the right seems to be kaput! But I haven't tested much yet. I hope it's gone forever.