"That's probably mostly hype to sell generators."

Well, yeah.
I certainty wouldn't classify it as a scientific paper.
For instance I have serious doubts that plants could reduce CO2 as low as 100 ppm. Perhaps they claim this because they relying on reports "from the field" which haven't been measured accurately [rumor and inability of measuring devices to give accurate numbers], though it might be possible that some plants are capable of lowering CO2 that low.

"Actual experiments with adding CO2 to plant growing areas have had mixed results."

Mix results in terms of how much an increase in growth will result from different plants, you mean?
It seems to me very likely that if you have plants which are at least not well ventilated to provide somewhere around +300 ppm CO2 plants could seriously suffer from this lack of CO2.

"There are so many other variables that have a greater influence on growth that often the CO2 variable gets lost in the noise."

In a natural environment in which there is a shortage of water, freezing temperatures, not mention disease, animals eating the plants, etc what you say could be somewhat true. But in a greenhouse or even open area crops, I think adding CO2 during daylight hours would invariably give significant increase in crop yields.

"As with everything else relating to climate, the system is so complex that simple answers should be suspect."

Well, do commercial greenhouses take measures to make sure there is enough CO2?

Google: CO2 enrichment + greenhouses
443,000 hits.
first one:
http://homeharvest.com/carbondioxideenrichment.htm
quote:
"SAMPLE RESULTS FROM CO2 ENRICHMENT STUDIES
BIBB LETTUCE
By adding CO2 to the atmosphere around the plant, a 40% crop increase was achieved. Whereas previous crops averaged 22 heads per basket, lettuce grown in the increased CO2 atmosphere (550 ppm) averaged 16 heads of better quality per basket.

CARNATIONS
CO2 levels to 550 ppm produced an obvious increase in yield (over 30%), but the greatest benefits were earlier flowering (up to 2 weeks) with an increased percentage of dry matter.

ROSES
The addition of controlled carbon dioxide provided a remarkable improvement in blossom quality, number and yield. Plants consistently produced many more flowers with 24 to 30 inch stems. Average yield was increased by 39.7%.

TOMATOES
Work in experimental stations has shown that crop increases of as much as 29% have been obtained by increasing the CO2 concentration. More desirable firmness and more uniform ripening are also observed."

Next one:
"Numerous CO2 -enrichment studies in greenhouses, growth chambers, and open-top chambers have suggested that growth of many plants could increase about 30% on average with a doubling of the atmospheric CO2 concentration, in the short-term (up to a few years, to most). However, the applicability of such work to the growth of plants outdoors, under natural conditions and for longer periods, has been seriously questioned. The free-air CO2 enrichment (FACE) approach represents the state of the science in exposing plants and ecosystems to levels of atmospheric CO2 that may be reached during the coming 50 years."
http://cdiac.ornl.gov/programs/FACE/face.html

Next one:
"When there is no CO2 supply and insufficient air refreshment in a greenhouse, the plants may consume more CO2 than comes into the greenhouse. This makes the CO2 drop to a level below the normal outside level of 340 ppm. Such a condition with a lower-than-outside CO2 concentration in the greenhouse is called CO2 depletion. If the CO2 level is as low as 200 ppm, photosynthesis is reduced to only half the photosynthesis rate at the normal CO2 level of 340 ppm. CO2 depletion causes the plants to be unable to utilise the good light conditions, and when this situation lasts longer, growth and production will drop dramatically."
http://www.hortnet.co.nz/publications/science/n/neder/co2_nr1.htm

Now, Tom what do you have in mind?