Nice challenge :-)
I have looked at your post, maybe you could try this:
1) Add an extra column to your BOM table, call it "WT Length"
2) Using your own relations, slightly edited, and show "x" in the "WT Length" column:
IF extract(asm_mbr_part_number,3,2) =="WT"
x = (asm_mbr_length_ft *...