Depending on the size of the can, you should be able to reach in the hole for the can and drill through a beam if necessary. Some construction will mount the drywall on furring strips that run perpendicular to the beams, in which case you can snake the wire under the beam. As for patching the ceiling, it really depends on the texture. If it is a popcorn texture, then you can find some popcorn spray at the paint supply. Other textures are usually slurried on which you can do by thining down joint compound and using a brush or trowel. It may take some work, but you can get it to blend well enough. Once painted, no one but you will notice.