So now that I’ve had a chance to revisit the iMac, I have some really good news!
First, thank you @tobiasBora because your tips pointed me in the right direction. Turns out, the only thing I needed to do differently was have the isight.fw file copied to $out/lib/firmware/isight.fw instead of the subfolder that my previous configuration used. I got that snippet from someone trying to install some Broadcom Wifi firmware and I clearly didn’t adapt it quite right on my first attempt. 
For anyone trying to get this to work themselves, this snippet of my configuration.nix file now looks like this:
hardware.firmware = [
(
# Copy the Apple iSight Webcam firmware into the appropriate directory
pkgs.runCommandNoCC “isight-webcam” { } ‘’
mkdir -p $out/lib/firmware
cp ${./firmware/isight/isight.fw} $out/lib/firmware/isight.fw
‘’
)
];
(This assumes it can find isight.fw in /etc/nixos/firmware/isight/isight.fw)
After a rebuild:
$ dmesg | grep iSight
[ 1.432201] usb 1-4: Product: Built-in iSight
[ 35.017409] usb 1-4: Found UVC 1.00 device Built-in iSight (05ac:8501)
$ l /run/current-system/firmware/ | grep isight
lrwxrwxrwx 1 root root 86 Dec 31 1969 isight.fw.xz → /nix/store/mdsf41r66y9vqpxzmz5k9kgd9r16pnvv-isight-webcam-xz/lib/firmware/isight.fw.xz
$ l /dev/video*
crw-rw----+ 1 root video 81, 0 Nov 21 14:03 /dev/video0
crw-rw----+ 1 root video 81, 1 Nov 21 14:03 /dev/video1
And most importantly, the “Camera” app in Pantheon can now access the built-in webcam!