Of course works ExpressInstall only when there is a Flash player > 6.0.65 installed, but when no Flash player is installed, it fails silently (in the name of unobstructive flash etc). But I want the browser to prompt to install flash. In the old days, when you used the embed tag, it would show this prompt. But with SWFObject, I can't find any option to force this prompt. All I can think of is placing some embed code inside the if (!so.write('flash')), but that is not much of a clean option to me.
swfobject isn't set up to use this - your best bet would be to create a page that has the 'old school' object/embed tag on it (the ones that flash outputs when you publish your swf) and direct them to that page to install the plugin.