C-class
Thanks for the idea RichardMEL! I didn't think about it since it's usually 800 baht extra over the full Y fare. However, it seems to be a little cheaper in this case...
BKK-Khon Kaen (KKC), Phitsanulok (PHS)
CWEB10 costs 1965 Baht (1250 LH miles)
WSAT costs 1000 Baht (625 LH miles, provided this fare qualifies)
BKK-KKC-BKK-PHS-BKK-KKC-BKK in W would thus be 6000 Baht, 3750 miles
BKK-KKC-BKK C, BKK-PHS-BKK W would be 5930 Baht, 3750 miles
Another option would be
BKK-CNX-BKK in C, CNX-HGN-CNX in Y, 6880 baht, 3750 miles. Could do CNX-BKK on a 747 in F so that makes this option look decent as well.