Hi!
Here is the tanto I had for a long time that I sent for shinsa in old polish that was Oei, mumei and received Tokubetsu. Same school originally attributed to the katana. So it is possible as long as the sword is Ubu and from an important school such as this one.
http://www.nihontoantiques.com/fss507.htm
In regards to why this was attributed to Nagato Akikuni, consulting the Nihonto Koza and seeing a few examples, 2 katana, 1 wak and 1 tanto, I can see where Tanobe sensei would draw the conclusion. The Hamon generally hoso-sugu with shallow notare, itame with nagare and masame and O-Hada. Tons of Ji-nie. All examples I saw were on point to my little knowledge. When I bought this sword, I thought it was obvious Sa school, O-suriage and late Kamakura or Nanbokucho, but Tanobe said it was ubu. That makes sense due to the fact that this would make this an uchikatana, deep sori with short nakago exemplified by nakago of this particular smith and others of this time period.
Anyway just my thoughts.
Matt