This has happened here before when a faculty member on the search committee found something in an applicant that was perfect for an ongoing speaker series. The applicant didn't quite fit the position as well as other candidates but was obviously pursuing research that meshed perfectly with the theme of the speaker series. So an invitation was issued.
This is exactly the scenario I had imagined.
Thanks for the replies - I'm glad to hear that it's not that common as I've never heard of this happening before. The seminar isn't for another month but it will be interesting to see how it turns out.