Use a KeyProperty and point at another entity.
Apologies if I am confused and reading more into your question than is there, however the following is what I'd tell any of my friends working in GAE for the first time:
When you use an object database like a relational database you will be disappointed. Google do offer SQL support if that is what works best for your application.
A good rule of thumb would be when showing a page for an entity if it requires more than one .get() (apart from a global like a Config or similar) then be very clear that you need this. So for instance I'd be looking at this shared entity and thinking do I really need to update it globally - does it belong as a entity in its own right - does it have enough unique fields of its own? If so then split it out and reference with a KeyProperty. You pay a penalty for two gets per page whereas there is no real penalty for being inefficient sith storage (sorting data more than one) generally the reverse is true of SQL.
Also worth remembering that StructuredProperty is just to add structure to a property (e.g. dot notation for convenience) technically it's just like having normal properties added to your main entity.
Hope this helps and again apologies if this is something you are already aware of.