I've done it with Matrix and the script many times. However it's not 100% reliable and these days it seems like Priceline is the only one that picks it up consistently. Used to do it with TK Premium Economy fares (when those were still a thing) quite often mixed with a return in Business class.

Use advanced routing code to force your desired airline, then in the box next to it F BC=x with x being your targeted fare class.

ITA-Matrix-PowerTools - Userscript for Orbitz/DL/UA/AA/BA/CZ/IB/LA/LH/LX/TK