I tried this last week, and it failed.
Went to the Costco page, filed out the form, got my coupon, and went to the recommended BMW auto dealer in Portland with a list of ~$2,000 in parts with BMW part numbers.
It confused the counter kid, who grabbed an older parts guy. He looked at my list, looked at the Costco paper, and told me:
1) They can't order motorcycle parts, period.
2) The Costco discount was only good for parts AND service done at that dealer, period.
He didn't budge after a brief rebuttal, so I realized that it wasn't worth arguing with him, and I left.
BTW, the Countryside Promo15 code still seems to work.