please don't get me wrong: there's nothing wrong with Mouseposé and I 
wouldn't discourage anyone from using it. In fact, I believe that 
Boinx have done a very good job with this tool.

The only thing to keep in mind, and that's unrelated to Mouseposé, is 
that enabling the support for assistive devices will enable all apps 
to gain access to all keyboard input being made without having to be 
root. It's a convenience function - and the only alternative to 
providing that is to run that application as root - which is even 
less desirable from a security standpoint as a single bug in such an 
app could *potentially* open up unlimited access to any aspect of 
your system. So Apple simply made a compromise with providing this 
option. The alternative of providing a white list for apps allowed to 
access the global keyboard event stream would certainly have been 
better than an "all or nothing" option, but then again: nobody's 
perfect.

The icon popups would show the name of the action, so, since the 
modifiers are port of the name of the action, they'd also get 
displayed this way.

The virtual keyboard already has actions for cmd, shift, alt and ctrl.

Best regards, 
Felix