Using native reporting functionality, some elbow grease, and lots of time, one approach could be to create a list report that has a view with the name of the project as the first column and then one column for each of the 255+ choices with a formula that returns 1 if that choice is used and 0 if that choice is not used, which you can then visually inspect (or save to Excel and then total and/or filter) to find the "all zeroes" choices. If that sounds tedious and horrible to you, then you're understanding correctly.
Depending on how unique the values of these 255+ choices are (across all parameter choices), you might be able to adapt my Another EXISTS story from The Twilight Zone: Mind. Blown. 💥 technique to instead retrieve the usage count of each 255+ choices....and (even better) restrict it to only those that do not exist (zero use).
And finally (which is what caught my eye), this is an excellent Business Rule to add to a future version of our Health Checker solution, and invite you to email me at firstname.lastname@example.org if you'd like to discuss it in more detail.