Oh I just found this in an article about Paris Hilton:
"An immigration official at the airport said that Hilton might actually have been able to pass the border if she had
applied for an entry permit earlier, instead of trying to do so the day after pleading guilty in Las Vegas.
Though Japan takes a tough stance on drug offenses, the bans do not always last and can sometimes be overcome. Soccer icon Diego Maradona was barred from entering the country in 2002 during the World Cup finals due to his multiple past drug arrests but was eventually given a 30-day visa as a "special delegate." Even though several members had drug issues in their past, the Rolling Stones were also allowed into the country after years of struggles at the border. Former Beatles member Paul McCartney was arrested and deported in 1980 for marijuana possession when he was caught with the drug at the Narita airport."
This is promising!
and I found this weird form on the embassy's website. A letter of Guarantee? Kinda sound like you can vouch for someone??? There's no information on it though...
http://www.us.emb-japan.go.jp/j/download/GL_E.pdf