I actually thought about it, but it's a total mess to parse :( Think of posts like "We're sorry we can't sponsor H1B at the moment, but we might get you a VISA for our London office".
On the other hand, once you're already browsing New York City, good ol' ctrl-f for VISA will probably serve you well enough.
Back when the web looked (to me) like it was going to move to XHTML2 rather than HTML5 data encoding using Microformats looked promising, http://microformats.org/wiki/job-listing. One doesn't hear "semantic web" much nowadays though.
As a sugestion for next feature, I'd recommend a selection for visa sponsorship or not.