The data is exchanged via a temporary xml file: previously to calling exec the tcl script writes the data in an xml file in /tmp/ and then calls the ReportLab script giving it the filename on the commandline. The ReportLab script then parses the xml and generates the pdf. This sounds a bit complicated - I tried first to pass the data via stdin, but could not make it work with non us-ascii characters (umlauts always showed up as weird two-char combinations).
No need for scary dynamically generated scripts here - actually I am not sure what you meant by that?
Running python and ReportLab from inside aolserver would be much more efficient I guess, but for now the approach above works fine.
You can check out a demo of that setup here: http://tsinger.com/cards - an online business card generator.