Have you tried to inner join the Account entity with an alias? like this:
SELECT AdministrationPerson/FullName AS Fullname FROM Administration.Person AS AdministrationPerson INNER JOIN AdministrationPerson/Administration.PersonAccount/Administration.Account AS Administration_Account
I done some reading up on Inner Join and based of your example I was able to get it right. ( Had trouble getting the inner Join query to work with the rest but I've figured it out.
select OpportunityName as Name, SalesStage as Stage, LastUpdated, FullName as Rep from Administration.Account as Account INNER Join Account/CRM.Opportunity_Account/CRM.Opportunity where CRM.Opportunity/SalesStage != 'Lost'
I have a few more to do so I'll see if I understand what I've done lol.