@Simpleton I'm very skeptical of an Italian recipe coming over. If it was Dutch or German, perhaps, but there wasn't a huge Italian presence in Japan before the turn of the century. Especially in Yamagata, where Isabella Bird specifically mentioned in her book that pretty much all the "foreign" goods for sale like wine, whisky, tinned meat, etc, were all inferior quality imitations.
I'm tempted to say it's made up but given how much research Sassa Taiga puts into the story, there must be some truth to it somewhere. I have a suspicion that he's got some very obscure reference books that are out of print.
Regardless, it's impossible to say for certain what exactly the egg wine is or was meant to be without either finding a first or second hand source, or asking the author himself.