You need to explain how to do these at the same time:
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...you could run the diesel engine at it's best BSFC rpm all or most of the time and just vary the load rather than rpm...
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Best bsfc is for a certain load range. Diesels tend to be less efficient at full power. But a diesel engine also tends to be heavy, comparatively, especially if you only run it really close to best bsfc:
Volkswagen 1.5L diesel
Here best bsfc occurs at about 1/2 full power. And full power bsfc is about 75% as good as best bsfc, like the difference between 40 mpg and 30 mpg.
What I mean is that for the power range you want, the engine is twice as heavy as you need. I don't think this leads to the best economy.
My father and I did this project long ago when parts were really hard to get and batteries were crap. Our car was a 1970 Vega chassis. We had a 30 KW diesel generator set and a 45 HP AC motor from an elevator. And 300 pounds of lead batteries. We could just maintain 50 mph on the Grapevine grade. 0 to 60 in about 30 sec. On a flat course the diesel would run a few minutes until the batteries were charged (also boiling) and then shut down for 20 or 30 seconds. Big pulses small glides. Fuel economy was a little worse as a hybrid than it has been as a gasser.
But you can do much better today. If I could do it again I'd use an Atkinson engine and a PM alternator, and lithium batteries, natch. Built from the ground up, a series hybrid is the best route to high fuel economy.
-mort