As far as I have been able to determine, you're correct, the On Screen Keyboard ALT key only "sticks" with a single key in the numeric keypad. Thus, you can't use the ALT+nnnn method of typing special characters (e.g., ALT+0189 for ½). You would have to use the Character Map utility instead.
You can, however, use multiple-key combinations with the On Screen Keyboard if the first two are CTRL+SHIFT.
For example, click CTRL, then click SHIFT, and then click HOME, and the text from the cursor location back to the top of the document will be selected. Or CTRL+SHIFT+ESC to start Task Manager.