I don't think you can have a command work on all workspaces simultaneously, but I think you can use the workspace number instead of current. Internally fluxbox starts numbering workspaces with 0, but for the user display numbering starts at 1.
If you use the workspace number, you may be able to use the MacroCmd to execute the commands in series - never used MacroCmd for anything other that a command (move, resize, xterm etc ...) though.
the man page for fluxbox-keys may give you some inspiration for what you're attempting.