Yes it would be possible to use the amavisd-new and Google Safe Browsing Lookup API specifications to write an e-mail scanning plugin that would "scan" all URLs in e-mails. The amavisd-new spec is written in Perl, so it would probably be easiest to write the plugin in Perl as well, but not necessary as according to their documentation.
As Andrew mentions, this type of scanning would likely be most useful if integrated in to a larger framework, like SpamAssassin, where it can be a contributing factor to the classification of incoming (or outgoing, in high security environments) e-mail.