I assume when you say "I beam wood" you are talking about truss joists, aka TJI's or BCI's. If the underside of the joists are exposed, I'd just screw the unistrut to the bottom cord. If you span at least 3 joists I wouldn't sweat the the bottom cord ever separating from the web of the joists, at least not if all you're lifting is corny kegs. As I envision what I think you are suggesting building, you're planning to cantilever the hoist over the keezer, so you'll need enough weight on the opposite side of the mast to counter the weight of the keg.
If you can make your base wide enough to straddle your keezer then your build can be much easier, all you need is to extend the base beyond the reach of the hoist arm and put casters under the rig. Lift you keg, roll it over the keezer, and lower away. I'm not convinced unistrut is the ideal material with which to build such a hoist, but it'll work.