Just to follow-up, I flew BKK-KKC-BKK on TG for 2,400 THB on a W fare. I did receive LH M&M credit:
27/05/2004 KHON KAEN-Bangkok TG 0047 M + 500 + 500
exec. bonus (25% of status miles) + 125 + 125
25/05/2004 Bangkok-KHON KAEN TG 0040 M + 500 + 500
exec. bonus (25% of status miles) + 125 + 125