I think as long as you have them in net pots the roots hanging down will be ok. If you are concerned get river pebbles and put in, but once the roots grow into it don't be pulling the pots out for the pretty root pix.

This all assuming you will have a couple cycles a day.

You don't NEED anything else in the pipes though adding a medium that will hold some moisture would lessen the frequency of waterings. Since the roots only hold so much moisture, you'll need to flood the system more often.
Also, unless the pvc is really heavy, you'll need to do something to keep light out of the pipel

High temps are what will rot your roots. Without any mass around the roots to hold the cool temp of the reservoir solution, the roots will quickly become the temperature of the outside air. The extent to which that is a problem will depend upon hot how it is.

I don't see a problem with this design if you specify a frequent watering schedule as has been already suggested, and including in the night.

The comment about algae is really the one that troubles me, you will need to better lightproof this or face elevated temperature due to the incoming radiation fraction converted to heat, and slimy algae in there thriving on its light, throughout the pipe and enveloping the exposed roots. Unfortunately the algal layer will take care of all the water retention you are concerned won' t happen, as well as block oxygen stagnate air unless there can be some airflow inside= bad conditions where the fungi will take over. If only it were so easy just to use this sort of unfinished PVC especially on sunny days in Florida. Nothing a little wrap or dark/light paint can't fix IMO.

I love reading all the messages from the USA I see the last one was Friday morning early am , while here it is 17.30 pm on Friday, so I get to read all the messages while the rest of the USA are sleeping. I love the responses from Pupilla Charites, it means even though I have been doing hydroponics for 40 years I have so much to learn. I generally buy my nutrients pre mixed , however after reading so many postings in this forum, I went out and bought the powdered A and B mixture , 2 x 20 litre drums and my big stirring stick. I have not done this for 30 years, but now I am happy to make my own brew.