Water Bear,
There is a relatively quick and easy way you could go, that doesn't involve reactors, but is more limited than reactors.
In an early post, you said that you wanted the program to delete all entities that are related to a call out, if that call out was deleted. Well, okay here's what you could do. You could store the entity names as XDATA embedded into the callout entites. And then what you could do is build a command called DUCT-DELETE or something like that, which would mine the XDATA, find the entites with the enames, check for their exsistence, and delete them. Doing it this way would save you some major headaches when it comes to creating reactors, but in the same respect allowing you to simulate the behavior in some minor ways.
You follow what I'm saying? The list would then be part of the drawing, and not slave to the drawing session.
Although on second thought, enames might not get you there, I don't remember if they're persistent in the drawing. But I know with 2004 and up that Handles are so that might do it...
Anyway, there ya go. I hope this helps you along a little...