I am not sure what you are thinking
100 grams is quite a lot...
If you are making a diamond out of that carbon 14, it would be huge- 500 carat....
Not sure what you mean by 'encase'...? I assume the diamond simply has a high isotopic percentage of carbon-14- diamond is simply crystalline carbon, you could make a diamond that is rich in carbon 14....
Again, the example you cite has a power of 0.2 milliWatts, yest it lasts a long time, but 0.2 milliwatts simply isnt a lot, and as I said, scaling it up is not easy...
Its neat, it might power a sensor , or some other EXTREMELY low power device for a long time.... its just not useful for devices that need significant power...