Not sure if you found out how to access an in memory feature class, but it took me quite a while to figure it out. And it was all because you need to call the build method of the schemabuilder object first. I doesn't exist until you call Build.
Then it is as easy as *** memGeoDatabase.OpenDataset(Of FeatureClass)("Deeds") *** memGeoDatabase here is just a GeoDatabase object that is my in memory geodatabase.