I am happy to say this requirement formula worked:

For the workshop: requires factions { roman, } and building_present workshop or factions { roman, } and not building_present hospital and not building_present library

Also, I tried replacing "and building_present workshop" with a hidden resource called "test", hidden resource which I haven't added to any region, and that worked as well:

requires factions { roman, } and hidden_resource test or factions { roman, } and not building_present hospital and not building_present library

The result is a less confusing code line and also a less confusing requirement description in the building browser. It will now tell you that the workshop requires library and hospital because it's not coded to display the "not" in front of them. It's still not seamless but at least you can see the buildings in the browser.