This, sadly, is not sufficient for me. The Browser-Window is resizable and I can not assume that everybody has the window big enough. At least as I'm displaying user-content that may have a length of several pages...
I currently solved it that way:
1) use body-text to replace "</html>" with "<script>window.print();</script></html>"
2) save the message body again (same filename)
3) force the webgadget to reload using SetGadgetText(webMessageID, "file:///" + strBodyFile.s)