Probably the same likelihood as a spammer deliberately constructing a custom spamming bot just for your website... but good point - It'll be difficult if the spammer posts mangled URLs as part of their spam.
I imagine you could use some idiosyncratic scheme; lots of different ways you could do it. Do blog spammers have time to customise their scripts for specific sites? Is it worth it? My guess is they tailor them to different types of popular blog software. And then have to hit a very large number of blogs. Sort of like email spammers have to send out enormous quantities of email.