I suppose you could use
unistrut. That would be quick and cheap. I don't see a problem with lagging it straight to the rafters as long as you use enough fasteners per strut and don't use bolts so big that they'll split the wood. As far as
safety cables go, you might be able to weld (pre-rig) or
bolt forged eye bolts to the
unistrut next to where you plan on hanging lighting fixtures (assuming that you can't get a
safety cable around the rafter). That said, I'm not exactly sure how people usually
safety a
fixture to
unistrut when it is flush with a solid surface. I do know that I regularly see distribution transformers and
unit heaters hanging from
unistrut and threaded rod with no provisions for
safety cables at all.
Depending on the situation, you could also suspend pipe (even if you only suspend it 3-4"), but I still think
unistrut would be best as it is cheap, designed for overhead suspension, and will get your lights as close to the ceiling as possible.
You may also need some big, heavy-duty washers between the
fixture's
yoke and the
unistrut nut. This will allow you to pivot easier while reducing the surface area for compression against the strut, as well as to reduce the risk of the
yoke getting bound up in the strut
channel.