I was there in 2006 so this info is probably out of date.
That said those “touristic” tour companies can also organise individual entry for multiple days with an official guide.
Back then I used a travel Company called Sam. Try contacting them through this web page
Contact me if you have any questions.
On the flip side. I’m currently trying to organise access to the Fukushima Exclusion Zone. I don’t suppose you would know how I would do that or know a fixer in the area?