A local brewey (Cigar City) makes an "oatmeal raisin cookie" beer. Talking with them at their tasting room one day and they told me they literally condition some of their brown ale, after fermentation, by throwing in a bunch of oatmeal raisin cookies into a barrel.
So adding bloody mary mix to a keg... should work?
Post production modification is an interesting topic, even if I probably wouldn't drink a bloody mary flavored beer...