To pass stuff to TeraTERM, you could use command line parameters when calling ttpmacro.
To pass stuff back, you could have TeraTERM write a file and then have Autoit read that file. This could be the log file, or some file whose name was passed to TeraTERM via the command line. This file could be as simple as a bunch of "VariableName=VariableValue" statements. You could then #include this file to set all the variables in AutoIt.
Another possibility would be to have TeraTERM put up a messagebox and then have AutoIT read and close the messagebox. This would cause at least a flicker of the messagebox window to the user, though.
You could also pass back some limited info back and forth by using the title bar of the TeraTERM window - see settitle/gettitle.
Hope this helps and that I understood your question correctly.