Hey Kite, unless you want the light for being able to see a Solatube is made of a uv stabilized polymer. Sunlight within the 200-400nm range does not get past it. This is done to protect the Solatube materials and to protect everything in the room which is being lit by a Solatube, such as interior paint and furniture so it won't bleach out as it would if exposed to direct sunlight.
Since a Solatube system won't emit wavelengths to benefit vegetative growth could it help in flowering? Not based on any studies that I've been able to find. The Solatube system is designed to act as diffusive wide area lighting system whereby the light collected on the roof is directed towards a ceiling diffuser often times by a series of tubes that bend and cause the sunlight to reflect against inner surfaces which further reduces intensity. Once the light makes it to the ceiling mounted diffuser it is scattered to create light over a wide area. Of course the Red-FR spectrum's are where you want to see increases in PPFD, not decreases, so whatever R-FR does pass through a diffuser it is of such low PAR intensity, as measured at the canopy, it's benefit to the plants are going to be negligible at best. I've never heard anyone make the claim that adding these systems would eliminate the need for artificial sunlight, allow one to reduce the wattage of the current systems in use or improve flowering by being a supplement to their current indoor lighting systems.
Think about it from this perspective. Adding a Solatube system would be such an obvious solution that there should be hundreds of thousands of these installed in grow rooms around the world. Solatube is consistently bombarded with questions about how well will their products work for indoor gardening? From my conversations with them they are coy in their response by not saying they will or will not work for this application. Solatube wants to sell product, but they also will readily acknowledge the UV stabilization characteristics of their products, leaving you to decide if you wish to purchase them for whatever application you plan on using their products for. I can't help but think that if Solatube systems could point to their products being beneficial to the indoor garden environment that they would be all over the marketing of their systems for exactly this application.
As far as your statement goes that the sun makes the greatest source for plant lighting I would disagree as it is too broad a statement. I think you would agree that for many of us growing outdoors is not even an option. But when it is an option, plants grown outdoors are subject to time of year, region, varying sunlight intensities, pathogens, theft, interdiction, transportation, etc., considerations that make crop reliability and repeatability part of an unknown economic equation until such time that the crop is successfully harvested, bagged and tagged.
I do agree that when growing indoors I may not equal the sun's intensity, again this too is based on region, time of year and weather conditions, but when growing indoors I will have year round security and control of the photoperiod/spectrums which allows me to continuously provide my crop with its optimum DLI while optimizing all aspects of the indoor environment. By reducing the plants stress levels and needs to fight off pathogens I've created those conditions whereby I can expect high quality and yields, with repeatable values, without having to traipse up into the mountains to maintain an outdoor crop.