Breakpoints are not that hard to implement.  Bob Hinkle did a
version a few years back for VisualWorks.  You can find the
code in http://st-www.cs.uiuc.edu/ftp/pub/Smalltalk/st80_vw/Breakpoint20.st

Bob's package not only lets you put a breakpoint anywhere, it
lets you do it for a single object, so you can trap the send of add: 
to one particular OrderedCollection and not have it affect any others.  
There is a commercial breakpoint package that does something similar.
I assume both packages work the same.  Bob's package worked by 
recompiling the method with a "self halt" in it, but not changing the
source.  He let you put a breakpoint on a single object by using
lightweight classes and giving an object with its own breakpoints
its own class.  I am sure that the details of how to do this will be 
different for Squeak, but the main idea should be the same.

-Ralph