Thanks for posting. You are correct in thinking that the print script step is not compatible with IWP. For verification, if you edit a script in ScriptMaker (or create a new one), there is a checkbox at the bottom of the left hand corner of the script window that indicates Web Compatibility. You'll find that printing is an incompatible script step.
The language PHP has printing functionality, but FileMaker scripts that include non-Web Compatible script steps will not run if the FileMaker database is hosted using the PHP Site Assistant.
Let me know if there is anything else I can clarify for you.