Concrete roof weight holding capacity

Concrete is a great building material because it has very high "compressive strength"; it resists being crushed, and so can support a very heavy load placed on top of it.

However, it has very poor "tensile strength" and "shear strength"; it can be "pulled apart" relatively easily. This means that hanging stuff from underneath usually doesn't work very well, because the hanging load will "pull" on the concrete. While the relatively light weight of something like this would not normally be a concern, the fact that you'll be putting something very precious and delicate in it makes me very wary.

If you want to try it anyway, forget the hook that came with the cradle; it's for screwing into a wood support member like a ceiling joist or beam, and will not work at all in concrete. You will first need to drill a hole in the concrete. This will require a concrete drilling bit (probably several of them; concrete is not easy to drill through and will dull bits very quickly) and possibly a hammer drill if it's treated or hardened concrete.

Then, you will want to install a concrete anchor. These anchors are designed to "crush" as you screw a bolt into them, expanding a part of their frame into and against the concrete to lock it in place. Here's one that should work for you:

Its 5,000-lb rated load may be a bit of overkill, but you don't want this to fail, EVER, so go for broke. You can even use some construction adhesive or epoxy to further "seat" the anchor in the hole.

Then, after any adhesive or epoxy you put in has had time to set and cure, you just hook the cradle to the D-bolt on the anchor (this may require an S-hook, climbing carabiner or threaded quick-link) and you're done.

There is a concrete that I have used, and it is regular concrete, and perhaps it can have gravel in it, but the type I have installed did not use any gravel/aggregate. Dry pack is what I have learned to know it by, and stone setters use it for building walls and laying flagstone sidewalks. I have used it for shower bases over PVC liner and coated it with Redguard for waterproofing. That is where I learned that it took on water real fast. I was to dampen the dry pack slab I set 2 days before. What water I poured on to move around just fell into the concrete, did not stand at all. I even had a decent troweled surface.

The mix is the same, just add enough water to get the concrete to form a ball when compressed in your hand, and I mean just enough, too much will screw it up.

This may be worth a shot, but be careful, this may clog too from atmospheric dirt, decayed debris settling in etc.

The pipe since it is covered with concrete can be black corrugated drain tile or perforated sch 35 set in a gravel bed. Holes in the pipe are slits in the black drain tile and gravel size will not matter. In the Sch 35 the holes are larger so you do not want to use a small gravel there, #57 crushed stone or something larger than 3/4" washed gravel depending on your area. You may want to consider using a different type of area drain. The way you describe "shower drain", sound like it get clogged pretty easily. Replacing the drain you have with something that has a much larger grate or trap included that allows debris to collect, but not clog, may do you well too.