Apparently Mountain Lion 10.8.2 Safari "supports" Passbook passes, in that it will preview and offer to sync ".pkpass" files to iCloud for the user's configured iPhone to load into Passbook.
However, I haven't found any documentation on how to get the browser to load such a file, other than the user explicitly downloading such a file.
Ideally, there would be a <link rel="alternate" type="application/vnd.apple.pkpass">
or similar tag so a supporting (and suitably configured) browser could pick it up and offer it. I can't find any mention of such a feature.
Other than that, the user would need to be directed to "click here" to download a pkpass and hope to heck they knew what it was and what to do with it. Otherwise, it's an opaque blob in their Download folder.
Obviously I don't want to be explicitly sniffing the User-agent for iOS 6 or OS X 10.8.2 and above, as this would not also check if the user had iCloud configured, and also precludes support for PassWallet and similar apps for Android and the like, along with other supporting browsers.