Is it possible to generate a PDF file using ReportLab in such a way that the front page (and possibly table of contents) are excluded from the page numbering done by Platypus?
Yes. The first example on chapter 5 of the user guide, SimpleDocTemplate has two hooks:
doc.build(Story, onFirstPage=myFirstPage, onLaterPages=myLaterPages)
Just change myLaterPages definition to conditionally print the page number.