I believe that in the standard Windows widgets that if you press alt on its own, it highlights the menu bar, and allows you to use "regular" keys to access those menu items. I'm pretty sure it's been this way since at least Windows 3.1. It used to be that the key to press was always underlined in the menu bar. Under more recent versions of Windows, it underlines the character only after you press the Alt key.

As far as I know, this is completely separate from defined accelerators, which may exist for menus as well as for individual menu items. I may be wrong and the default is to set up both methods for menus. Assuming I'm not wrong, though, I can see where they might have defined one and not the other. I'm sure that a Windows developer would be more readily able to answer that.

That said, though, the fact that your tablet is unable to press Alt on its own means that it is not linking into the input stack at a low enough level. Any key you press generates a KeyDown event and a KeyUp event, including Alt, Control, etc. Later in the stack, the OS reinterprets those events into new events like Alt-G, Ctrl-C, etc. If your driver cannot send an Alt-KeyDown event, that's the fault of the driver; it's certainly possible. Maybe you can find a different macro application to stack on top of it.