Update: After reading the manual a bit more, I discovered that by going into the “Customize Features” tab in the menu, it is possible to change the function of the AE-L/AF-L button to “hold” the exposure after the button is pressed once and then release it only after the button is pressed a second time. This would allow the user to essentially disable the “AutoExposure” feature while filming and adjust the parameters of the shot completely manually. Filming at 30fps with a shutter speed of 1/60, f-stop 5.6, and iso 100, I got this shot:

It is much less grainy and appears to be better exposed – I’d consider it a success!
I suppose the moral of the story here is to read the manual in detail when your equipment gives you trouble. ☺
Thanks so much again for your help! Happy filming!
-Anthony