Yes, it is possible, but you need to understand the basics of function scope.
a=myapp
a.buttonpushedcallback(a)
would be wrong in all contexts. You either put a. at the front or you pass a in as an argument, you don't do both because they amount to the same thing so you would be passing it in twice, as the first two arguments with this syntax.
You use
a.buttonpushedcallback( )
or
buttonpushedcallback( a )
if a is an object of a class and buttonpushedcallback is a method of that class. For a method that is not part of the class you just call
buttonpushedcallback( )
If you really want you can have a method that is external to the class that still takes an object of it as first argument:
buttonpushedcallback( a )
but you cannot use the a.buttonpushedcallback syntax here. Quite why you would want to do this I don't know, since if it takes the class object as argument it should be a method of the class in general, but it is possible.
For a callback though you generally need to pass in two arguments representing the source and event data though also for it to be recognised as a valid callback signature.
