Why? What are you trying to achieve? You're talking about some pretty intrusive techniques. Unless you're developing a code analyser of some sort, you probably should be looking at a better way of doing what you're doing.
To capture the acceptance of a method (assuming you mean the action that happens when you press alt+s), you insert a bit of code into PluggableTextMorph>>accept.
To determine which instance variables are being assigned to, you'll need to somehow look at the bytecodes. They're not too hard to analyse, but it can be a bit of work. Alternatively, maybe the refactory browser can help, or maybe you can look at the intermediate code that the compiler generates.
The bytecodes are described here:
http://burks.bton.ac.uk/burks/language/smaltalk/goldberg/blueb003.htm. They're in the "Blue book chapter 28" if you need to Google it. You'll want the "store" bytecodes.