>I've got a report where I'd like to take one of the fields and (based on it's value) substitute in a different value. I'd really rather do this in the report instead of manipulating another view/array/whatever. I don't think the report writer supports this, but I'm hoping there is some cool trick to do it.

It supports it without any tricks, just type 'MyFunction(Parm1,Parm2...)' in field expression box.