Originally Posted by jerry a. laska

Correct. If you want to MFU a BA segment the ticket must be purchased from BA either on the website or over the phone, or if purchased in the US, from a BA preferred travel agent. So, to rephrase, if you purchase the ticket from anywhere but the US the ticket must be purchased from the BA website (impossible for rtw tickets) or over the phone from BA (purchasing from a travel agent in EUR or Africa will not allow you to MFU). The ticket will, of course, be issued on BA ticket stock. I doubt that your questions to AA about MFUing a BA segment will result in any consistent answers because the AA people will necessarily be guessing or trying to apply their own logic. You should direct questions about MFUing to BAEC.