Hi Brian. If this is going to be an on going calculation then you should consider calculating them server side: Performance Equation, Totaliser, ACE, ... Then you reference the data like any other tag.
Or...do you have AF? You could make use of the PI Point DR with the Formula DR, then access the data via the AF Dataset in Processbook.
Or...use PI SDK to evaluate performance equations on the fly.
You are right in that you cannot reference datasets from within a Dataset. From memory there is no clean way to obtain the value from a PI Expression Dataset in VBA.