I am a Japanese travel fan and I would like to show my personal opinion.
In Japan, the train travel is very expensive as compared to that in Europe. For example, Paris to Amsterdam by Thalys is only 35 euro if you book early (maybe 2 months in advance). Tokyo to Shin-Osaka is about the same distance and the price is more than 100 euro even if you book in advance. In general, the price of many things are more expensive in Europe than in Japan, but the train travel by Shinkansen is one exception. I think this is due the government policy: In Europe (I donft know it can be applied to all countries in Europe), government pays substantial money for the train (maybe basically of the rails) but in Japan, government pays less money for train systems after the JR was privatized. Therefore, to increase foreign tourists, the government decided (I think) to introduce Japan Rail Pass system.
At this time, Japan Rail Pass does not have black-out dates. Tokaido Shinkansen is surely for business, but the busiest dates such as new year days, Obon days and the Golden week are for tourists. Japan Rail have some discount tickets for Japanese such as gFull-moon passh for elderly couple (including homo-sexual these days), but this pass has black-out days (entire summer season and the busiest days as written above). In this meaning, foreign tourists are allowed huge freedom as compared to Japanese locals. This is because of the government policy to increase foreign tourists.
According to this article, the number of Japan Rail pass issued in 2016 was around 800 thousands. https://tabiris.com/archives/japanrailpass/
Due to the increase of foreign tourists these years, it might be around 1 million per year. If all the pssses holders use the pass twice for the Tokyo to Kansai leg, 2 million foreign tourists should use the pass to take Shinkansen per year. This is not so many as compared to the total foreign tourists coming to Japan (about 30 million). Because many are from east Asia (Korea, China etc.) and you can come to Japan very cheaply and if you want, you can come to Japan simultaneously quite easily.
If Japan want to earn money from foreign tourists, long-term tourists using Japan Rail Pass are welcomed. So it is not a bad idea to give them some more insentive, such as Nozomi on Tokaido Shinkansen
Tourists using Tokaido Shinkansen are 165 million which is about 100 times more than the estimated JR pass holders using this line.http://company.jr-central.co.jp/company/achievement/financeandtranspor...
So it would have not so much impact of the congestion. In most days Tokaido Shinkansen has vacant seats in Nozomi as well as slower Hikari and Kodama. As for black-out days, I donft think it is necessary for the JR pass holders. In very busy dates, the seat will rapidly be sold out and most of the JR pass holders can not take it even it is open for the holders. One problem is multiple and un-used booked seats. At present there is no penalty for these, so I have heard some JR pass holders do this. The JR companies can not sell these seats to other people which is problematic. Multiple booking at one time at the ticket booth will take time which is also a problem.
Therefore, my idea is:
Open Nozomi to JR pass holders.
Small gbooking feeh for each booking.