I was about to suggest that... regardless of the advice/opinions/experiences shared in this forum, I think there's enough fog around what kinds of itineraries BA will accept MFUs on, and what kinds they won't, that the real answer to your question may only ever be ascertained by a call into BA. And even then, only if you happen upon someone who knows what they're talking about.

And if in the end you can actually get what you're after, well, it's probably well worth the $20 offline ticketing charge...