I'm a bit confused about the BA scheme for upgrading. Any help would be appreciated. Here's the plan:

SEA-LHR (AA) LHR-NBO-LHR (BA) LHR-SEA (AA) booked at aa.com

Would like to accomplish:

1. Paid fare on AA so I get miles and can use my EVIP upgrades.
2. Travel LHR-NBO in either business or first. (have 100,000+ BA miles)
3. Avoid the "luxury tax" for RT SEA-LHR by making a connection to NBO.
4. Avoid those terribly high BA taxes and fees.

I'll be able to upgrade the AA portion with EVIPs. I'd like to upgrade the BA flights either with money or BA miles. At BA.com it appears there's an option to purchase and upgrade (25000BA Miles plus $ 1,389.47) or the Upgrade mileage calculator which appears to cost 150,000 miles for F.

First, can this be done on a fare issued by AA? Second, is the above calculation correct? If it is, it doesn't seem to be a good value as a RT fare SEA-NBO is ~ $1,700 or SEA-LHR-SEA ~$800 + 100,000 miles for F LHR-NBO-LHR.

I've contacted AA about using a paid fare connecting to a BA award LHR-NBO in F but they will only allow two of the three following conditions on the same ticket:

1. Paid fare SEA-LHR
2. Using EVIP to upgrade AA segments
3. Award LHR-NBO

Thanks