To clarify, the mods I have installed include Bugfix, AricsExpansion, Marriage and More, and Constants. According to the wiki, the only prerequisites to “Hollow Nipples” is that the character in question can’t already have hollow nipples, and that her breasts must be Average at minimum (I try to keep them all at Incredible at minimum, so that’s not the reason it’s not showing up.) Does anyone have any advice on why the option doesn’t appear in my laboratory?

When mods conflict the results can occasionally cause small parts of the game to malfunction, but more often than not the game simply crashes. The mods you listed and in the order you listed are generally safe. The notable exception currently being for AricsExpansion versions since 1.9, the Marriage and More mod will need the fix from the AricsExpansion Discord. That said, there are no conflicts that would be expected to impact the laboratory. Instead it's simply a normal AricsExpansion bug.

In order to add the increased body part sizes for things like breasts, AricsExpansion added 6 new arrays of size names to replace the single array of sizes in the original game. However, in the laboratory the single comparison for breast size was never changed from the original array to the new array, thus any new sizes would register as a size -1 and hollow nipples would be unavailable. The fix for this is simple, open up ".../files/scripts/laboratory.gd" in the program folder with a text editor and change the line:

if globals.sizearray.find(person.titssize) >= 3 && person.mods.has('hollownipples') == false:

to:

if globals.titssizearray.find(person.titssize) >= 3 && person.mods.has('hollownipples') == false:

Save the file and restart the game. Since this only changes the program file, any time you apply mods it will be erased and need to be changed again.