Currently we are not able to reference an annotation applied to a navigation property.
In the following example the expression <Path>Supplier/@Core.Description#Bar</Path> references the annotation of the entitytype supplier. The annotation at the navigation property "Supplier" is not referencable. As of today the expression <Path>Supplier@Core.Description#Foo</Path> is not valid.
<EntityType Name="SalesOrderItem">
<Key>
<PropertyRef Name="soi_ID" />
</Key>
<Property Type="Edm.String" Name="soi_ID" />
<NavigationProperty Name="Supplier" Type="X.Supplier">
<Annotation Term="Core.Description" Qualifier="Foo" String="Item is supplied by" />
</NavigationProperty>
</EntityType>

<EntityType Name="Supplier">
<Annotation Term="Core.Description" Qualifier="Bar" Path="ID" />
<Key>
<PropertyRef Name="ID" />
</Key>
<Property Type="Edm.String" Name="ID" />
</EntityType>

<Annotations Target="X.SalesOrderItem">
<Annotation Term="Core.LongDescription" Qualifier="First">
<Path>Supplier/@Core.Description#Bar</Path>
</Annotation>
<Annotation Term="Core.LongDescription" Qualifier="Second">
<Path>Supplier@Core.Description#Foo</Path>
</Annotation>
</Annotations>

See also Ralf's comment at https://tools.oasis-open.org/issues/browse/ODATA-320