From some great suggestions and discussions I found several alternative approaches to the one that was my initial focus. THAT IS FANTASTIC. This project is still in "Step 1 -- Identify and Investigate (gather data)". It means that I should not be limiting options at this point. That comes only in the next step of Evaluation.
Two more options I began researching for this project was the possible use of SiPMT devices instead of a vacuum tube PMT. The second option was to use a different scintillation crystal.
On the surface (I will detail more when I post the project plan spreadsheet), it seems that an SiPMT of the quality and sensitivity needed would be too expensive. It would have some fantastic benefits though. But if I cannot afford it, then it becomes "unobtainium".
Using the BC-408 organic crystal that I have was based on an initial goal of simply having a more sensitive counter. However, based on further research and the project discussions it seems that actual gamma spectrography may be possible. As with the SiPMT, that would be based on my ability to obtain a proper NaI(T) scintillation crystal for a reasonable price. This option is looking to have potential. I have located a 40x20 crystal for a reasonable price. My actual requirement would be a 40x30, but the loss of 33% in capture volume would be made up by the increased sensitivity.
One commenter suggested transmitting the audio from the analyzer real-time to be captured and processed at the ground station. That is an easily implemented item and would solve the problem of speed for any digital downlink.
Great stuff! Onward, and upward.